NBSI or Not Blue, Shoot It, is a rule of engagement which most of nullsec, lowsec and wormhole space follows. In it, every capsuleer is a valid target unless they have blue standings of some form.

NPSI or Not Purple, Shoot It, is a rule of engagement that only a few organisations such as Spectre Fleet or Bombers Bar follows. In it, every capsuleer is a valid target unless they are in the same fleet as you, regardless of standings.

NRDS or Not Red, Don't Shoot, is a rule of engagement that basically only Provibloc follows. In it, no capsuleer is a valid target unless they agress first or have red standings. Not even neutrals can technically be shot under this rule.