ipip-412: explicitly forbid identity cid blocks

src/ipips/ipip-0412.md

@@ -116,9 +116,16 @@ behavior is unspecified, and the CAR response includes an arbitrary list of
 blocks. In this case, the client should assume `n` as the default, but ignore
 duplicates if they are present.
 
-<!-- TODO: do we need a  parameter for inclusion of identity CIDs?
-It seems to be only relevant in Filecoin due to legacy hiccup:
-https://github.com/ipfs/specs/pull/330#issuecomment-1274106892 -->
+:::warning
+
+The specified parameter does not apply to virtual blocks identified by identity
+CIDs. CAR responses MUST never include these virtual blocks. The parameter in
+question is meant to control the behavior of non-virtual blocks in the
+response. Therefore, it does not have any effect on virtual blocks, and they
+should never be included in the CAR response, no matter if present, or what
+value is set.
+
+:::
 
 ### Signaling in Response