diff --git a/ges/ges-extractable.c b/ges/ges-extractable.c
index 0c75986d5d..c3efec93f7 100644
--- a/ges/ges-extractable.c
+++ b/ges/ges-extractable.c
@@ -21,9 +21,25 @@
 /**
  * SECTION: gesextractable
  * @title: GESExtractable Interface
- * @short_description: An interface for objects which can be extracted from a GESAsset
+ * @short_description: An interface for objects which can be extracted
+ * from a #GESAsset
  *
- * FIXME: Long description needed
+ * A #GObject that implements the #GESExtractable interface can be
+ * extracted from a #GESAsset using ges_asset_extract().
+ *
+ * Each extractable type will have its own way of interpreting the
+ * #GESAsset:id of an asset (or, if it is associated with a specific
+ * subclass of #GESAsset, the asset subclass may handle the
+ * interpretation of the #GESAsset:id). By default, the requested asset
+ * #GESAsset:id will be ignored by a #GESExtractable and will be set to
+ * the type name of the extractable instead. Also by default, when the
+ * requested asset is extracted, the returned object will simply be a
+ * newly created default object of that extractable type. You should check
+ * the documentation for each extractable type to see if they differ from
+ * the default.
+ *
+ * After the object is extracted, it will have a reference to the asset it
+ * came from, which you can retrieve using ges_extractable_get_asset().
  */
 #ifdef HAVE_CONFIG_H
 #include "config.h"
@@ -86,12 +102,12 @@ ges_extractable_default_init (GESExtractableInterface * iface)
 
 /**
  * ges_extractable_get_asset:
- * @self: The #GESExtractable from which to retrieve a #GESAsset
+ * @self: A #GESExtractable
  *
- * Method for getting an asset from a #GESExtractable
+ * Get the asset that has been set on the extractable object.
  *
- * Returns: (transfer none) (nullable): The #GESAsset or %NULL if none has
- * been set
+ * Returns: (transfer none) (nullable): The asset set on @self, or %NULL
+ * if no asset has been set.
  */
 GESAsset *
 ges_extractable_get_asset (GESExtractable * self)
@@ -103,12 +119,22 @@ ges_extractable_get_asset (GESExtractable * self)
 
 /**
  * ges_extractable_set_asset:
- * @self: Target object
- * @asset: (transfer none): The #GESAsset to set
+ * @self: A #GESExtractable
+ * @asset: (transfer none): The asset to set
  *
- * Method to set the asset which instantiated the specified object
+ * Sets the asset for this extractable object.
  *
- * Return: %TRUE if @asset could be set %FALSE otherwize
+ * When an object is extracted from an asset using ges_asset_extract() its
+ * asset will be automatically set. Note that many classes that implement
+ * #GESExtractable will automatically create their objects using assets
+ * when you call their @new methods. However, you can use this method to
+ * associate an object with a compatible asset if it was created by other
+ * means and does not yet have an asset. Or, for some implementations of
+ * #GESExtractable, you can use this to change the asset of the given
+ * extractable object, which will lead to a change in its state to
+ * match the new asset #GESAsset:id.
+ *
+ * Returns: %TRUE if @asset could be successfully set on @self.
  */
 gboolean
 ges_extractable_set_asset (GESExtractable * self, GESAsset * asset)
@@ -123,15 +149,19 @@ ges_extractable_set_asset (GESExtractable * self, GESAsset * asset)
   if (iface->can_update_asset == FALSE &&
       g_object_get_qdata (G_OBJECT (self), ges_asset_key)) {
     GST_WARNING_OBJECT (self, "Can not reset asset on object");
+    /* FIXME: do not fail if the same asset */
 
     return FALSE;
   }
+  /* FIXME: shouldn't we check that the extractable-type of the asset
+   * matches our type? */
 
   g_object_set_qdata_full (G_OBJECT (self), ges_asset_key,
       gst_object_ref (asset), gst_object_unref);
 
   /* Let classes that implement the interface know that a asset has been set */
   if (iface->set_asset_full)
+    /* FIXME: return to the previous asset if the setting fails */
     return iface->set_asset_full (self, asset);
 
   if (iface->set_asset)
@@ -142,9 +172,18 @@ ges_extractable_set_asset (GESExtractable * self, GESAsset * asset)
 
 /**
  * ges_extractable_get_id:
- * @self: The #GESExtractable
+ * @self: A #GESExtractable
  *
- * Returns: (transfer full): The #id of the associated #GESAsset, free with #g_free
+ * Gets the #GESAsset:id of some associated asset. It may be the case
+ * that the object has no set asset, or even that such an asset does not
+ * yet exist in the GES cache. Instead, this will return the asset
+ * #GESAsset:id that is _compatible_ with the current state of the object,
+ * as determined by the #GESExtractable implementer. If it was indeed
+ * extracted from an asset, this should return the same as its
+ * corresponding asset #GESAsset:id.
+ *
+ * Returns: (transfer full): The #GESAsset:id of some associated #GESAsset
+ * that is compatible with @self's current state.
  */
 gchar *
 ges_extractable_get_id (GESExtractable * self)
diff --git a/ges/ges-extractable.h b/ges/ges-extractable.h
index fc99b65bc0..281ef09b95 100644
--- a/ges/ges-extractable.h
+++ b/ges/ges-extractable.h
@@ -42,18 +42,72 @@ GType ges_extractable_get_type (void);
 
 /**
  * GESExtractableCheckId:
- * @type: The #GType to check @id for:
- * @id: The id to check
+ * @type: The #GESExtractable type to check @id for
+ * @id: The ID to check
  * @error: An error that can be set if needed
  *
- * Returns: The ID to use for the asset or %NULL if @id is not valid
+ * Method for checking that an ID is valid for the given #GESExtractable
+ * type. If the given ID is considered valid, it can be adjusted into some
+ * standard and returned to prevent the creation of separate #GESAsset-s,
+ * with different #GESAsset:id, that would otherwise act the same.
+ *
+ * Returns (transfer full) (nullable): The actual #GESAsset:id to set on
+ * any corresponding assets, based on @id, or %NULL if @id is not valid.
  */
 
 typedef gchar* (*GESExtractableCheckId) (GType type, const gchar *id,
     GError **error);
 
 /**
- * GESExtractable:
+ * GESExtractableInterface:
+ * @asset_type: The subclass type of #GESAsset that should be created when
+ * an asset with the corresponding #GESAsset:extractable-type is
+ * requested.
+ * @check_id: The method to call to check whether a given ID is valid as
+ * an asset #GESAsset:id for the given #GESAsset:extractable-type. The
+ * returned ID is the actual #GESAsset:id that is set on the asset. The
+ * default implementation will simply always return the type name of the
+ * #GESAsset:extractable-type, even if the received ID is %NULL. As such,
+ * any given ID is considered valid (or is ignored), but only one is
+ * actually ever set on an asset, which means the given
+ * #GESAsset:extractable-type can only have one associated asset.
+ * @can_update_asset: Whether an object of this class can have its
+ * #GESAsset change over its lifetime. This should be set to %TRUE if one
+ * of the object's parameters that is associated with its ID can change
+ * after construction, which would require an asset with a new ID. Note
+ * that the subclass is required to handle the requesting and setting of
+ * the new asset on the object.
+ * @set_asset: This method is called after the #GESAsset of an object is
+ * set. If your class supports the asset of an object changing, then you
+ * can use this method to change the parameters of the object to match the
+ * new asset #GESAsset:id. If setting the asset should be able to fail,
+ * you should implement @set_asset_full instead.
+ * @set_asset_full: Like @set_asset, but also allows you to return %FALSE
+ * to indicate a failure to change the object in response to a change in
+ * its asset.
+ * @get_parameters_from_id: The method to call to get the object
+ * properties corresponding to a given asset #GESAsset:id. The default
+ * implementation will simply return no parameters. The default #GESAsset
+ * will call this to set the returned properties on the extracted object,
+ * but other subclasses may ignore this method.
+ * @get_id: The method to fetch the #GESAsset:id of some associated asset.
+ * Note that it may be the case that the object does not have its asset
+ * set, or even that an asset with such an #GESAsset:id does not exist in
+ * the GES cache. Instead, this should return the #GESAsset:id that is
+ * _compatible_ with the current state of the object. The default
+ * implementation simply returns the type name of the object, which is
+ * what is used as the #GESAsset:id by default.
+ * @get_real_extractable_type: The method to call to get the actual
+ * #GESAsset:extractable-type an asset should have set, given the
+ * requested #GESAsset:id. The default implementation simply returns the
+ * same type as given. You can overwrite this if it is more appropriate
+ * to extract the object from a subclass, depending on the requested
+ * #GESAsset:id. Note that when an asset is requested, this method will be
+ * called before the other class methods. In particular, this means that
+ * the @check_id and @get_parameters_from_id class methods of the returned
+ * type will be used (instead of our own).
+ * @register_metas: The method to set metadata on an asset. This is called
+ * on initiation of the asset, but before it begins to load its state.
  */
 G_GNUC_BEGIN_IGNORE_DEPRECATIONS
 struct _GESExtractableInterface