The B channel is a neutral conduit for bits; so the meaning of the bits flowing in the channel must be understood by the users at each end.

If the two users are not using the same "language", they won't understand each other and so there is no meaningful communication. Imagine making a telephone call where the person who answers doesn't speak the same language as you. You can't even apologise for disturbing them!

The same is equally true of D channels. There are various "dialects" of the signalling protocol. You must be using the same dialect as your network in order to successfully communicate with it.