Tonight I got idea that this is probably BBcode ([mod] tag BBcode) with additional use of Abe's mod for controlling which usergroups can use it. I'm talking about this
mod.
As I see, user couldn't even see BBcode tag, which is good to know.
Thanks for confirming this information to me. This really helps, as I already knew about Abe's mod, but never did actually used it. Only problem is that I'm on vB4, and MobileRead is on vB3, so I'll need a bit of testing before I start using Abe's mod (it's a bit outdated and it's possible that it doesn't work completely or at all with vB4).
Anyway, this is great and simple idea from the owner Alex, I wish I had it before. I even asked some coders if they can make mod with similar functionality.
EDIT: Just tested on my test site, [mod] BBcode is visible to users when Abe's mod is used, but isn't parsed, so you probably use some other mod, probably custom made.