During blueprint transition users can add or modify the value of a lookup field.

If the lookup field already contains a value, and during transition the agent selects a different value, the older value will be overridden. Selecting an appropriate value for the lookup field during transition can also be mandated.
Note that only the primary field can be added as a lookup.
Kindly refer to the help doc: Lookup fields in Blueprint transition for more information.
Availability: Professional and Enterprise edition.
DCs: All
PM: @Akhil Vijayakumar
Regards,