The abbreviation N.B. is short for the Latin nota bene, meaning "note well!".
It is used as a firm command, reminding people not to forget something!
e.g. N.B. Always use capital letters at the start of sentences!
a.m. & p.m., cf., e.g., et al., etc., i.e., N.B., P.S., Q.E.D., q.v., viz.
ad hoc, de facto, modus operandi, non sequitur, pro rata, quid pro quo, sic, vice versa.