Override Customer or ShopUser

Hello

I need to override shop to add document uploads at registration (for professional account)
So i have successfully overrided customer entity with a new ManyToMany to a new entity for doc upload (vichupload bundle).
Everythings works but i discovered (too late) there is a Customer Entity and a ShopUser entity.

According to best practices, which one should be linked to my docs ? im a bit confused because in doc customer represents guest user but this entity is linked to address, order …

Regards