In ArcFM 10.2.1a and 10.2.1b, when Add Related is selected on a GU in the Design tab, only one related item is available for selection, despite there being one or more other relationships configured. For example, the POLE WOOD CLASS 3 45ft GU in the image below also has a relationship with JOINTUSEATTACHMENT. It is properly configured with the DESIGNUNIT model name and should also be shown along with Assembly.
This is caused by bug CLS-58132.
This bug is fixed in ArcFM 10.2.1b SP1. A work-around does not exist for this bug in the affected versions.