By default, your action bar has one of each type of skills, for example one slot is for primary, one slot for secondary, another for Defensive and another for magic, etc By turning on Elective mode, you can assign any combination of skills to your action bar. So for example you can choose to have only primary skills. or have two primary, and two secondary. etc.
hmmm interesting didnt know about this lol and I'm almost through Normal This is a lot better than the slot presets
Elective mode is an option in the game that allow the gamer to put more than one skill from the similar category. These category distributed in class. Keep Elective Mode off will boundary your skill preference combinations.