TAD Docs

You can toggle the class lock using the same menu-item. Once unlocked, TAD would allow you to edit/create objects anywhere. Note that for a class sub-tree to be locked, the parent of the said class MUST be “Project” AND that class should not have any objects of its own. It the class has further sub-classes; those should be made only in an earlier locked session.

See also how to lock a sub-tree

Important
Locking or unlocking of a branch is now restricted – It is available ONLY if you have set the COLLABFOLDER field in the Project Properties of that project. That means TAD must be convinced that indeed this project is being used for collaboration