Those are called dialogue state names.
There are practically no restrictions on them, except in interjections - things that start with INTERJECT or I_C_T. If you are writing an interjection, you MUST be using a unique name, prefixed with your own prefix.
(By the way, I see you have registered a prefix? This is well done! So, all your I_C_T's have to start with BI(but please keep in mind that you'll be using this prefix for all your mods, and it might be wise to prefix it with your NPC's name, like I do with Xan's variables, which all start with O#Xan)).
Outside of interjections - in dialogue, banters, flirts and romance talks - feel free to use whichever labels you like! Heck, even a, b, c, d,e is fine!
But, and it is an important but: these dialogue states must have unique names within the file. It doesn't matter if some other mod has a dialogue state a1.1, you can have it, too. But you cannot have TWO states called a1.1 - you must use b1.1, or a1.2, or a2 instead.
I am using long, long dialogue state names, because I have thousands of those, and I don't want them to be mixed up. Also, sometimes I want the names to mean something(NotPC), so I immediately recall what the dialogue state is all about.