think of those slots as if they were a chain of stomp boxes. The order in which the sound passes thru the chain can produce different results.

if the track is MIDI (which has no sound until it passes through a synth) the first thing in the chain needs to be a synth (DXi or VSTi plugin)

Now that the MIDI has sound, the next slots would be things like reverb, EQ, compression whatever, in the order you want the signal to pass through


AUDIO tracks already have sound, so they don't need to pass through a synth. Therefore, on audio tracks you can start adding effects immediately... first slot could be anything from compression to phaser to chorus to distortion etc etc... up to the number of available slots.