In general, check
this link out. It's a great guide to Yanfly's action sequences!
:D
You have five kinds of actions available to you.
<setup action> is the stuff that happens before the "meat" of the skill, this is for stuff like the battler moving forward a bit, unsheathing their sword, casting animations, that kind of preparatory stuff.
<whole action> affects all the targets at once. This is for AoE stuff that should hit every target at once with the same effect.
<target action> is for individual effects on individual targets.
<follow action> is for cleanup after each individual targeting action, this is for things like removing IMMORTAL flags (so if your skill hits, say, three enemies one after another, you'd have a target action to do your attack on the first target, then a follow action to remove that target's immortal flags so if it's at 0 HP it dies, etc, then another target action for the next target etc.).
<finish action> is what you wrap up a skill with, this is generally for things like moving your battler back to its home spot, running WAIT actions, whatever else you want to have happen here. I make a point to put PERFORM FINISH, CLEAR COMBAT LOG and IMMORTAL: targets, false in a finish action at the end of all my skills so that it puts the user back at its home, removes the ability name from the top of the screen, and doesn't accidentally leave one of your targets unable to die.
:)