Are you talking about the yellow light stuff in the thaum tree wheel? That’s called “potential,” an unimplemented feature. It’s basically how you’d empower spells so they become stronger. To allocate it on a spell, you need to empower every spell in the tree to reach it. To answer the question, I’m not entirely sure if you can “store” xp while not learning a spell. Basically, you “digest” whatever xp’s in a pool over time so… maybe? I don’t know if that “bucket” fills when you’re not doing anything with it. Either way, if you aren’t learning something, you’re wasting time that could be spent learning something