A Stomp or preamp compressor effect (between the bass and amp)would most likely be achieved in Slot A, B, C, or D. A Studio compressor sound (between amp/microphone and mixing board) would likely be created in the X, MOD, or DELAY Slot.
Usually, I place the compressor as stomp B, right after the noise gate as stomp A.
First i used compression in the A-Slot, but now i mostly use the compression in the amp section
I usually use both First one in the A or B slot with fast attack (to cut the peaks, even out the sound and make it more solid) and the second one in the X or Delay slot as paralel compression (mix about 65 %) with higher intensity and slower attack (to improve sustain and smoothiness)
Normally I use it on the first two stomps (it depends if I have to put in also a noise gate). But it really change the comp behaviour if you put it after or before some effects or after a saturated profile, so it can be used after in a creative way
First one in the A or B slot with fast attack (to cut the peaks, even out the sound and make it more solid) and the second one in the X or Delay slot as paralel compression (mix about 65 %) with higher intensity and slower attack (to improve sustain and smoothiness)