Use another bundle-field as the title in the selected relation instead of NodeID and Title-field

Dear Guys,

thanks for the great tool!
My Problem is, that the selected relations shouldnt contain the nodeID + Title (as it actually is), but instead use another bundle-field (e.g. Type-No. = Textfield) as the shown title. (See attachement)
Can you tell me where the code is, or how to change that?

Any tips are appreciated!

Thanks in advance and regards,
Fab