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.