In some cases it doesn't work and makes Poser crazy, I have tested many years ago to discover Poser's format.
In many places in Poser files you can add things and lines and Poser will ignore these extra items as Poser4 ignores all the shader trees and new instructions.
In these case it doesn't matter if it begins with # or nothing, Poser will ignore it, but..., in some places you cannot put extra things and Poser will not ignore and will do wrong things and # solved nothing.
As # didn't work and Poser's files are in C style my guess was that comments must begin with // and it worked in any place.
Don't remember if /* ....   */ works too.

Stupidity also evolves!