There are four types of perfection attainable, and these only in certain Phases of incarnation: Self-Sacrifice (in Phases 2, 3 and 4), Self-Knowledge (in Phase 13), Unity of Being (in Phases 16, 17 and 18), and Sanctity (in Phase 27) (see AV B 95 & 100). Generally Unity of Being is used by Yeats to cover some or all of these, since it was the form that interested him most and which was personally possible to him.
A thing is "perfect" so far as it has emerged from the incompleteness of potency, in which all finite entities begin, and possesses the complete activity required by its nature for its proper perfection. Only the "perfect" is desirable or "good."
It is achieved when a created being completely fulfills the purpose for which it was created. A human being becomes perfect when he or she reaches full maturity of character, that is when actions of the body are in absolute harmony with God-centered mind, husband or wife is in full unity with their spouse, and humans are in complete unity with nature.