There was already a pretty... well-known regiment by the name of 1tesPmrs, so [1tes]Pmrs was the get-around. Also, there's no way once we joined the IIte (which was in the works for over a month before it happened publicly) that we could fit IIte_1tesPmrs_Rank_Name in, when it's actually a bit difficult for some people to fit their names after IIte[1tes]Pmrs_Rank_. It's all down to preference, I guess. If a regiment has a tag, like Pmrs or LG or QF or whatever, I find it better to use the brackets, like [Reg]blahblahblah_rank_name. The #[Rank]Name system is plain weird IMHO. Especially when it's like, oh, idk, 24th[Rank]_Name. It's just odd. lul.
The only instance where I don't believe in that is Grenadier regiments. Personally, I like 1er_Gren_Rank_Name, and Nr24_Gren_Rank_Name. It just looks oddly nice, and arguably better than [1er]Gren_ and [Nr24]Gren_.
If I lead a regiment that wasn't in an army and didn't have a distinct tag like Pmrs to separate us from the other 1tes', I'd probably go with the classic #_R_N.