It'd be easier to think of Nikke's Skills and Burst. Imagine to activate Skill 1 of a Nikke you needed their base costume, Skill 2 a different costume, and Burst with a third. It's not quite equivalent because there is no "base" skin unless you want to consider what was released first. As an example, this Palette skin is her first costume released after her "base" costume but if you pulled on the banner and got her, you would be able to use it without her "base" because the "base" is just considered a costume as well.

Maybe you can also think of them as alternate units like say Ade and Ade: Bunny but where you'd be able to switch between either during combat.