Well, for now the plan is that we just provide the API - it depends on the implementation how to store the data directly.
If you want to create a custom mod that adds a few new components you might want to think about how to store them. Since we probably wont have a meta-data system, but a component-based system I am sure that there’ll be an implementation (like NBT) that can add new data to entities.
In the end it really just comes down to you; Whether it’ll be a text-file, just fields or something else.