Where the fly angler places oneself so that they can make the most ideal presentation to a trout or to a location where a trout is likely to be lying.
As an aside relative to a stream's "regulars":
It is a dead give-away that an angler is totally unfamiliar with a specific water when they set-up right where the trout hold! There is no harm if they position themselves away from where the trout are since they will not spook the trout into hiding. BUT, if they position themselves at the least ideal presentation location (on the trout!!); the next angler has to rest the area for an inordinate amount of time before the trout return to their feeding or prime hold from which they where spooked.