Can someone please confirm my assumptions? Are these correct statements?

Attributes: You can define attributes (additional fields) for flexible and meaningful classification but they are limited to leads, opportunities, customers, and inventory items.

User defined Fields: The functionality of attributes is not available for all entities. For some of these entities, your organization can create user-defined fields. They will be displayed in an additional Tab “User Defined Fields”.

Custom Fields: Attributes and user defined fields can’t be placed at grid or line item level. You would need a custom field for that.