User Defined 1 External ID Type not showing up

any particular reason why I cannot select User Defined 1 for a partner’s external ID type?? This is essential for any ID’s with a qualifier of other than ZZ or 01. I’m stumped as to why this is not an option in the dropdown list. Thanks

Not sure I understand your question but
User Defined = ZZ
DUNS = 01
User Defined 1 = what ever you want it to be.