Under pure game mechanics, there is a "default power" associated with the array, which is where one assumes the array is at at the beginning of combat unless the player has explicitly set it elsewise. That may mean that players get caught with their pants down (if part of their defense is in a slot which is not being invoked) and/or having to spend their one switch at the beginning of their turn to get the power they want. Dynamic arrays can have a "default allocation" if they generally walk away with the points apportioned in a particular manner.
When there's multiple powers allocated to a particular array slot (an array slot could easily be "Blast 8, Flight 2" for a 20 pp slot in a standard array, and something like Super-Senses and Super-Movement carries several powers by its nature), there is little guidance in the rules for dynamic array slot use, but in 2E, there was official clarification that you have to invoke at least one rank of each
and many GMs will force you to be proportionate (so if a slot has Blast 5/Flight 5, they may make you buy them apace with each other). When invoking dynamic power slots, all Extra must be applied for the ranks used. That means all "flat" Extra points as well as any Extra associated with the ranks used.
Lastly, while the rules do not prohibit "nested arrays", many GMs are disinclined to them. An example:
Blast 10 [20 pp + 1 for the AE]
.. AE: Blast 6 (Burst Area) [18 pp of effect + 2 for the AE]
.. .. AE: Blast 6 (Cone Area) [18 pp of effect]
.. .. AE: Blast 6 (Shapeable Area) [18 pp of effect]
Personally, I think there are situations where nested APs make sense, such as an arsenal of weapons which includes a grenade launcher that has multiple options for grenades, but they could be abusive if used improperly.