Things You Should NOT Do With Discard

Don't try to bake actual sourdough bread with it. Discard is unfed and weak — it won't reliably leaven a loaf. That's why levain exists.
Don't use discard older than ~1 week from the fridge. It's not unsafe, but it'll be aggressively sour and may have separated significantly.
Don't use discard with visible mold or pink/orange streaks. Throw the whole jar out and sanitize. (This is rare in fridge-stored discard but worth knowing.)
Don't substitute discard 1:1 for active starter in a recipe that calls for active starter — you'll need to add a bit of commercial yeast or accept a much denser result.