What is the "witness reserved value" for BIP-141? 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?
It's a reserved value.

That means it is reserved for future extensions, but for now there are no rules about it. It's encouraged to not use it for anything, as that may conflict with later use.