here's an ntr and bss breakdown from a doujin publisher for you (i think i've posted this crap 5+ times here already):
NTR:
BSS:
"B – Boku ga
S – Saisho ni
S – Suki datta
“Boku ga Saisho ni Suki datta”, known as “BSS”, translates to
“I liked her first”. This subgenre has stories where the MC and the girl are
NOT a couple, but the MC has feelings for the girl. Often the MC is too cowardly to, or due to bad timing, hasn’t had the time to confess his feelings. Since the MC and the girl aren’t together,
there is NO “stealing” taking place if the girls has sex with another guy. Hence, a lot of hentai artists debated whether “Netorare ” would be the correct clarification of the “stolen-before-I-could-confess” trope. Thus, BSS came to be and is picking up a lot of speed."