Just imagine how wonderful that tart would be with crabapple jelly. Do you really want to risk it being a total dud by using, say, apricot preserves, or apple jelly, or one of the other mainstream jellies that wonderful cooks have been using to glaze tarts for decades, if not centuries?

No, we're sorry, but your recipe writer specified crabapple jelly for a reason, and we just can't allow you to make a time-saving substitution that will leave you bitter and resentful and give your teenage children another reason not to speak to you.

Crabapple jelly was specified either because your recipe writer is a total priss or because it is likely to be a little more tart than apple jelly, or both. So if you can find something like a currant jelly, a plum jelly, a quince jelly, etc., that would be fine. Apple jelly would be a fine substitution, though slightly less interesting in the end, while grape jelly would be our last resort.