I have no idea what a primed mirror is.

Yes, but you can also paint it on a gesso-primed canvas, or even on stretched paper. "universal primed' means: primed with gesso.

In that sentence, the word "primed" is the predicate (verb). (v) to make ready, prepare

The principal function of a pattern recognition system is to yield decisions concerning the class membership of the patterns with which it is confronted. In order to accomplish this task, it is necessary to establish some rules upon which to base these decisions. One important approach to this problem is the use of decision functions.

"Primed" can function as either a verb (past tense of prime) or an adjective (meaning prepared or ready).