want to understand the difference of getperceivedactors, getcurrentlyperceivedactors,getknownperceivedactors

hi,i am in trouble understanding the difference between UAIPerceptionComponent’s function
GetPerceivedActors
GetCurrentlyPerceivedActors
GetKnownPerceivedActors

i want to understand what those function acttually returns,i mean i know
they return the actors that are sensed but why three functions?
please can someone explain this thing or give me some link to any kinds of resources
that explains this in a detailed way.