The documentation says: “To use the part synonym feature, navigate to the parts screen via PARTS » View/Lists » List » [Find a part]. Once you have selected a part, click on the ‘Crosses’ link from the side navigation for the Part.”
I can’t find Crosses. I can find a Cross Part List (outside the parts screen), but it has no editing capability. I tried to find a config setting, but failed.
Here’s a friendly screenshot.