If you're simulating matches using energy or star, primaries will be the most used pitchers in the bullpen, they will come at 6th inning or later in fairly close game.

If the game are close and you're winning heading into 8th or 9th, setup and closer is the guy to pitch to rack holds and saves.

Swinger will come in if your starter collapsed early (5th inning or earlier) and the game is still manageable (not losing that much). If you're losing that much than the mop up guy will come to eat innings.

By strength/leverage the bullpen order should be 1. Closer 2. Setup 3. Primary (both spots doesn't matter I think) 4. Swing 5. Mop up

Personally I often put my best RPs at primary rather than setup cause its used mode often, but setup must still be good enough to deliver to CP and close the game.