Why Subform is not showing User field type and Notes field type?

Why Subform is not showing User field type and Notes field type?

In my simple task management system, i have a subform that has the following fields and types:

Service (lookup)
Assignee (users)
Notes (Add Note)
Due Date (date)

In the actual subform, only the Service and Due Date show up? Why? Where can I find documentations of the subform limitations? If these are indeeded limited, what's the recommended workaround?