Settings → Retirement plan defaults holds the two app-wide postures behind every plan on Retirement plan: assumed inflation (the today's-dollars deflator applied to every projected value and withdrawal) and Project to age (how far the simulation runs, above your own current age). They are edited here rather than on the page itself because they are the household's standing assumptions, not one scenario's what-if. A saved scenario still carries its own copy of both, set when you saved it, so changing the default here reaches a brand-new plan and does not silently move a plan you already saved.
The rest of the panel is a read-only mirror: birth date, filing status, state tax rate, and taxable embedded gain all come from your Household profile, the single home for facts about real people. A number here that looks wrong is a Household-profile fact, not a Settings knob, and the fix belongs there; every mirrored row says so in its hint.
Preview with an example on Retirement plan borrows none of this: it substitutes a whole sample household so nothing of yours is mixed into the demo run.