I’d recommend using DataStores, it sounds like you’re trying to save when someone completes something. If you need, there’s a quick tutorial here: Data Stores. Also, I’d recommend using ProfileService as it has automatic data loss prevention and anti-duplication measures, but do whatever feels right to you.
DataStores permanently store data in “keys”, but they can be associated with players by using their userid (e.g. using the key “User23847”). I’d recommend looking at the article.
I recommend ProfileService because it acts as a normal table, you never have to use :SetAsync or :GetAsync. Everything is automatic.