This setting determines which application has control over project information when synchronizing between Project Management and Microsoft Project.
If PPM controls all shared work plan information (Project Management-controlled mode), then any changes made in Microsoft Project are automatically discarded when project information is synchronized, as described in Publishing a Project Management Work Plan through Microsoft Project.
If Microsoft Project controls all shared work plan information (Microsoft Project-controlled mode), then work plan fields in Project Management are not editable, as described in Publishing a Microsoft Project File through Project Management.
If control is shared (shared control mode), then work plan information is determined by Microsoft Project, while actuals data is gathered by Project Management, as described in Using Both Applications to Track Different Project Data.
Note: While it is possible to switch modes during project execution, it is not recommended unless you are moving to Project Management control.