When I create a Basic Group constraint in PlanAhead, I can only specify one instance; this does not make much sense if that instance is not a level of hierarchy.
Is this the intended behavior?
A TNM constraint can be applied to multiple instances, so PlanAhead should allow you to add multiple instances.
This issue is scheduled to be fixed in a future release of PlanAhead.
In the meantime, use Constraints Editor to create TNM constraints in the current release.