@piggo I am looking at that spec of yours, I wouldn't break my head about it. The address is optional anyway and if I understand it right also superfluous (why would you have server device being able to respond to multiple device addresses there?). If I were you, I would model it as a regexp like `/[^!]*!\r`, or something like that. But you are right, the spec is rather cryptic in this regard. My hunch is that they are trying to say that the device address is "unique" in its entirety, i.e., only one device can have that address and it cannot happen that multiple devices would have the same. Like a MAC address spec (while we all know they are not that unique in reality, but that is an implementation detail). But it's a very clumsy formulation for an IEC standard.