Activities added using the Open Plan Add-In can be linked to a control account or work package, or not linked. An activity is linked, by populating the control account and work package linking fields, if it logically rolls up to a control account or work package (that is, it contains resources that are rolled up to form part of the cost or otherwise helps define the work under the control account or work package).
You can only add activities using the Add-In under the following conditions:
Add an activity that is linked to a control account or work package.
Add an activity that is not linked to a control account or work package as long as it impacts other activities which themselves roll up to control accounts or work packages.
Add an activity that is not linked to a control account or work package as long as there are other changes in the workflow that impact control accounts or work packages.