When exactly can you use combat actions? For example, I was playing an Eldar/Dark Eldar deck, and I had an army unit that could move to an adjacent planet as a Combat action.

Let's pretend he's at the first planet.

Can I use it at the very beginning of the combat phase, before mobility movements are made and before the battle actually starts at the first planet?

Can I use it after the battle at the first has started, but before ranged attackers make their attacks?

Can I use it directly before or after one of my other units has attacked?

Can I attack with it, then immediately move it to an adjacent planet while still exhausted, before my opponent gets a turn?

If the combat round is over, and everything is exhausted, and I'm deciding whether or not to retreat to HQ, can I move it to an adjacent planet at the same time as I move other units to HQ?

If my opponent attacks my unit, can I move it as a combat action before it takes damage?

What if he's at the 2nd planet, alone. Can he move while the battle at the first planet is going on?

You can use a combat action during any of the Action Windows marked on the flowchart for the game in the RRG.

It could not be used to avoid damage. If it was some kind of an Interrupt that might work depending on how it was worded.

When I played last night with my friend, he only brought over the base rule book and I couldn't find anything about Action Windows. Thanks!

You can download both books on the support page.

Found it, massively helpful, thanks.