what is (send from cache) in user manager?
What does coa stand for?
what is coa nak & coa ack?
Coa (in the context of Radius) means change of authorization, RFC5176:
https://datatracker.ietf.org/doc/html/rfc5176
Generally ack stands for acknowledged, and nak for not acknowledged, or if you prefer, accepted and refused/failed, example:
http://forum.mikrotik.com/t/hotspot-freeradius-coa-with-mikrotik-group-attribute-not-working-correctly/94653/1