If the axel is the second jump, it is a sequence, since you have to turn forward for the take off.
But axel lands on the back outside edge like all the other jumps, so axel-toe loop, or axel-loop are combos.
You have to stick something between it to do axel-salchow, so that is a sequence. (Basically, if the jump takes off from the RBO, for a CCW skater, then it can be a combo. Any other edge, it is a sequence, because you have to get to that edge.)
I guess if you were a really talented jumper and could do a lutz in the opposite direction, you could do axel-lutz, (you'd have to pick with the "wrong" side for a "normal" lutz, but if you land a CCW axel on RBO, you could take off for a CW lutz).