BIP-141 defines a
witness reserved value as part of a flexible commitment structure:
Double-SHA256(witness root hash|witness reserved value)
The purpose is to support commitments (i.e., data associated with a block) beyond the witness root hash. That part makes sense, but it seems like the value of
witness reserved value should be defined in BIP-141. For example, should it be omitted altogether when hashing the witness root hash? Should it be set to
0x00? Some other value?