Assistant Executive Officer (AXO)

In general, most positions will have an assistant, responsible for duties as assigned, acting when the Chief is off-duty or otherwise engaged, and filling in if the Chief's position becomes vacant.

In the case of the AXO, the character also has a Chief position of his/her own. This isn't necessarily Chief Operations officer. While Data was AXO and Chief Operations Officer, Dax was AXO and Chief Science Officer, Tuvok was AXO and Chief Security Officer, and Scotty and Trip were AXO and Chief Engineer.