The distinction in my mind is: References are not something you can directly use as a contribution. It needs a bit of “processing”. For example, a tangent discussion thread, could be a very helpful reference. But to use it towards the goal of the artifact / initiative it may need to be interpreted first.
On the other hand, a contribution is specifically for the the artifact / initiative. A PR that writes some of it’s code. But also discussion threads, if they’re specifically for designing the thing in question can be contributions imo.
For Initiatives that’s currently the case. All edge types flow cred in both directions. The weights do vary though.
Contributions for instance have a 1x forward 1x backward. Because contributions are for that initiative, it lends a lot of credibility to the initiative as well. References have a 1/2x from the initiative to the reference. And a 1/16x in reverse as this incentivizes initiatives to include many references, but doesn’t add a lot of credibility to the initiative.