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I want to use a line from copied geometry as a connection in assembly but can't because it's considered an assembly item and not a component item.
One option: Create a skeleton model in the assembly that defines all of the connection references needed. You can then use the skeleton to create and maintain the connections.
Another option: Create the copy geom in the component needed to set up the connection rather than in the assembly. This will be an external copy geom feature.
I think it's not because it's geometry in the assembly, but because it is a sketch. If it's a line, you can define an axis using the two points of the line, then use that axis to assemble things. I tried to use sketch geometry as a reference for assembly and it is not able to be selected.