The 2d Constraints tools are based on solving on a plane (2d). It would be good to expose and report this 'host' plane in the Properties Panel (Ctrl-I).
If there are multiple solving planes in the model (very likely) they should be numbered and re-name-able by the user to ensure legibility.
This should pave the way for the host plane to reference a model xy, xz, yz or an ACS.
This has been marked 'Will not implement'. It would be great to understand why. The inability to understand which plane the constraints are using makes using Mstn Constraints that much harder, error-prone. Most if not all MCAD apps provide good feedback on where the host plane is and provide naming.