diff --git a/src/tags/master_playlist/media.rs b/src/tags/master_playlist/media.rs
index f83d9b6..e30b079 100644
--- a/src/tags/master_playlist/media.rs
+++ b/src/tags/master_playlist/media.rs
@@ -111,10 +111,17 @@ pub struct ExtXMedia {
     /// essential to play.
     #[builder(default)]
     is_forced: bool,
-    /// An [`InStreamId`] specifies a rendition within the
-    /// segments in the [`MediaPlaylist`].
+    /// An [`InStreamId`] identifies a rendition within the
+    /// [`MediaSegment`]s in a [`MediaPlaylist`].
+    ///
+    /// # Note
+    ///
+    /// This attribute is required, if the [`ExtXMedia::media_type`] is
+    /// [`MediaType::ClosedCaptions`]. For all other [`ExtXMedia::media_type`]
+    /// the [`InStreamId`] must not be specified!
     ///
     /// [`MediaPlaylist`]: crate::MediaPlaylist
+    /// [`MediaSegment`]: crate::MediaSegment
     #[builder(setter(strip_option), default)]
     #[shorthand(enable(copy))]
     instream_id: Option<InStreamId>,
diff --git a/src/types/in_stream_id.rs b/src/types/in_stream_id.rs
index 6f1fd1a..7d36d6a 100644
--- a/src/types/in_stream_id.rs
+++ b/src/types/in_stream_id.rs
@@ -1,15 +1,26 @@
 use strum::{Display, EnumString};
 
-/// Identifier of a rendition within the segments in a media playlist.
+use crate::traits::RequiredVersion;
+use crate::types::ProtocolVersion;
+
+/// Identifier of a rendition within the [`MediaSegment`]s in a
+/// [`MediaPlaylist`].
 ///
-/// See: [4.3.4.1. EXT-X-MEDIA]
+/// The variants [`InStreamId::Cc1`], [`InStreamId::Cc2`], [`InStreamId::Cc3`],
+/// and [`InStreamId::Cc4`] identify a Line 21 Data Services channel ([CEA608]).
 ///
-/// [4.3.4.1. EXT-X-MEDIA]: https://tools.ietf.org/html/rfc8216#section-4.3.4.1
+/// The `Service` variants identify a Digital Television Closed Captioning
+/// ([CEA708]) service block number. The `Service` variants range from
+/// [`InStreamId::Service1`] to [`InStreamId::Service63`].
+///
+/// [CEA608]: https://tools.ietf.org/html/rfc8216#ref-CEA608
+/// [CEA708]: https://tools.ietf.org/html/rfc8216#ref-CEA708
+/// [`MediaSegment`]: crate::MediaSegment
+/// [`MediaPlaylist`]: crate::MediaPlaylist
 #[non_exhaustive]
 #[allow(missing_docs)]
 #[strum(serialize_all = "UPPERCASE")]
 #[derive(Ord, PartialOrd, Debug, Clone, Copy, PartialEq, Eq, Hash, Display, EnumString)]
-#[non_exhaustive]
 pub enum InStreamId {
     Cc1,
     Cc2,
@@ -80,6 +91,15 @@ pub enum InStreamId {
     Service63,
 }
 
+impl RequiredVersion for InStreamId {
+    fn required_version(&self) -> ProtocolVersion {
+        match &self {
+            Self::Cc1 | Self::Cc2 | Self::Cc3 | Self::Cc4 => ProtocolVersion::V1,
+            _ => ProtocolVersion::V7,
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;