For example take a look at T11363, if you are not a member of Community. "Edit Related Objects" is greyt out, you can't use any of the three options:
You do not have permission to edit this object. Users with the "Can Edit" capability: Members of the project "Community" can take this action.
But you can do it the other way round: Go to (a differential revision for example), and click "Edit Tasks", now you can add the revision. This is confusing, and strange, because you have the permission, you just need to use the other way to execute it.