diff --git a/transactions/0131.md b/transactions/0131.md
index 26766a6..0623348 100644
--- a/transactions/0131.md
+++ b/transactions/0131.md
@@ -168,6 +168,13 @@ The global scope (`FF0E`) ensures block announcements cross site boundaries. The
 group index `0xFFFE` is in the reserved control-plane range and is never used as
 a shard group for any `shard_bits` ≤ 12.
 
+The address is shown in ASM form (`FF0E::B:FFFE`), which under RFC 8815 is
+intra-domain only; inter-domain delivery across site boundaries uses the SSM
+address `FF3E::B:FFFE` (see [BRC-129](./0129.md)). The frame format, `HashKey`,
+`SeqNum`, and NACK path are unchanged across modes. Under SSM, receivers
+`(S,G)`-join using the block-announce source (the emitting proxy, `senderIPv6`);
+block-broadcast sources are distributed per BRC-129.
+
 ### Sequence Tracking and Retransmission
 
 BRC-131 frames use the same XXH64 hash-chain sequencing as BRC-124: