Joins allows us to combine data from 2 or more classes with common Primary-Foreign Key data. I dont think of any limitation on classes you can join until the time you have common identifiers in both tables. Hope this helps

Joining can be done only when the multiple tables have some common identifiers, like primary key in one table should be available as a foreign key in other table, Without the relation joining is never possible.  While designing Entity relationship these things to be considered for related tables.

"If RD is in Framework class and Framework  RS, I don't think we can join with Implementation Class"

Thanks

Joining happens with tables only, irrespective of framework or implementation classes, if those classes have some common identifiers in their respective  tables, they can be joined.