In some sense, there is no difference: if you look at the code, they actually are aliases of one another:

So they are synonyms at the database level.

At a semantic level, belong to implies a level of ownership. You can think of the cart as “owning” a line item. This is different than the relationship between a line item and a product… a product has an independent existence, and doesn’t “own” line items.

If this makes sense to you, then this is just one of the many ways that Rails helps you express your intent when defining a model. If it doesn’t - don’t worry to much about it, because ultimately these two terms are synonyms.