Demetrios1453
Legend
I don't see any way where "additional" does not include "custom" within its (very broad) definition. Custom backgrounds aren't in the PHB, and, thus, by any definition of the word I can think of, must be "additional" to the ones in the PHB.That's not text encouraging players to collaborate with the DM to create custom backgrounds. That's text encouraging players to ask the DM what background options the DM offers. There's no rule suggesting the DM make custom backgrounds available by default the way the DM in your hypothetical conversation does.
How exactly, in your opinion, are custom backgrounds not a subset of backgrounds additional to those in the PHB?