It is itself the tool, and uses the core rules as written.A reason/hook for people who don't plan on running the Realms to buy it. How does it expand the available rules or tools for D&D 5e?
I'm assumng the market to be a) those who are intending to either run it stock or convert it to their own purposes, and b) those who are simply FR fans and who will thus buy it with or without intent of using it for a campaign.
If it's to have baked-in rules expansions (new spells, feats, etc.) as well, there's room to fit 'em in; but to me those would be the first things to be cut for space as FR is in theory a generic (and thus, core-based) setting.