RDM Relationships that are NOT parent-child - how are you handling?

Hi All - 

Looking for design advice on tables that have related data that is not required (as in the case in the “relationship” where it’s parent-child and required).

Take the following example: cost codes that can be related to different hierarchies of “groups”: “departments”, “project groups”, “governance groups”.  Each of the organizational groupings have their own relationship to cost code - but not all of them are required to use cost code, which rules out using a parent-child relationship. Also, it’s not necessarily an Mn Relationship, because each “group” (at whatever level) can only have 1 cost code, if it’s related at all.

Additionally, we are not implementing any synchronization from our source systems due to security constraints - so things will be either manual or import by UI or plan if that makes a difference in terms of usage.

I’m thinking about creating a junction table to hold the related values - but that doesn’t seem ideal from a user/UI perspective, because then users would have to know to go to the “junction” table to create the relationship. Do-able (documentation, training, naming conventions of tables), but not ideal.
 

Any thoughts of how to accomplish?

Here’s a diagram that shows the use case above: