Vendor record must have Alma MMS ID in 001 field. If it does not match to an existing Alma record it will not load.
No inventory will be created.
If a record is open in the MDE it will not be overlaid.
The incoming record will overlay the Alma record, with certain field being protected. See list of protected fields here: https://wiki.harvard.edu/confluence/x/y1HKDQ