Can you import an attribute that can have multiple values, e.g.
if so, what's the format of the value in the import file?
I don't believe any Excel-based import supports multiple values at this time. You could submit an Enhancement Request to ask for that kind of feature, though: https://solutions.teamdynamix.com/TDClient/Requests/ServiceDet?ID=2148.
The asset importer that is available in TDNext in an asset app does not have support for multiple custom attribute choice selections for a single field, no.
Sr Support Consultant, CS