i think when you added setting "quality affect capacity (fluid wagon)" then we will need another option for storage tank ;).
(EDIT: StorageTankPrototype has no quality_affects_inventory_size ) i wonder if there is another way to add quality affect.
PS. consider add option to "quality affect roboport range (suply/construction area)
(EDIT2: hmm range also dont scale with quality in prototype properties).
Also what chages with quality affect roboport-equipment ?

all available prototype flags other than items scaling the max speed of fuel are given adjustement points. These are all the options they opened up to us so far. Other mods are using duplicate entity which replace your original quality entities in order to provide additional scaling.

I want to keep this within a tight scope, only modify prototypes and the qualities themselves. So if they add more prototype options later, I will add them as well!

To answer your question, this changes the ports of the roboport. So higher quality gains additional charging slots.