add note for encryption description limit

index.bs

@@ -17,6 +17,7 @@ Metadata Order: This version, !*, *
 <pre class="anchors">
 url: https://www.iso.org/standard/68960.html#; spec: ISOBMFF; type: dfn;
 	text: VisualSampleEntry
+	text: Sample Auxiliary Information
 
 url: https://www.iso.org/standard/68960.html#; spec: ISOBMFF; type: property;
 	text: bitr
@@ -29,6 +30,7 @@ url: https://www.iso.org/standard/68960.html#; spec: ISOBMFF; type: property;
 	text: pasp
 	text: sgpd
 	text: stsd
+	text: saiz
 
 url: https://www.webmproject.org/vp9/mp4/#; spec: VP9; type: property;
 	text: SmDm
@@ -619,6 +621,8 @@ Within protected samples, the following constraints apply:
 	    - <assert>A subsample SHALL be created for each tile that has a <code>decode_tile</code> structure whose size (including any trailing bits) is larger or equal to 16 bytes</assert>. <assert>If it is less than 16 bytes, per the rules above, the <code>decode_tile</code> structure is not encrypted and the corresponding bytes SHOULD be included in the [=BytesOfClearData=] field of a surrounding subsample, if any</assert>.
 		- <assert>All other parts of [=Tile Group OBUs=] and [=Frame OBUs=] SHALL be unprotected <span hidden>for cenc</span></assert>.
 
+NOTE: Version 0 of the 'saiz' box as defined in [[ISOBMFF]] can only describe [=Sample Auxiliary Information=] whose size does not exceed 255 bytes. In the case of AV1 streams that use a large number of tiles or a large number of frames per sample, when following the encryption requirements defined in this section the size of the corresponding Sample Auxiliary Information may exceed that limit. Until updates are made to [[ISOBMFF]], authors are encouraged to avoid reaching the limit.
+
 Subsample Encryption Illustration {#subsample-encryption-illustration}
 ------------------
 Figure #1 illustrates Subsample based encryption of AV1.