1. You are importing Gifts
2. You have valid BBDM Finder Numbers in your file
3. You map the column containing the Finder Numbers to: Gift Fields--->Finder Number
4. You have the checkbox in General Settings of your profile selected to "Use BBDM for Finder Number"
During the import, Constituent biographical data will be updated/created based on the data in BBDM.
It is important to note that any Biographical information mapped in the import profile will be ignored in this case, and the updates to existing records are automatic.
If all the conditions above are met, then the import can also create new constituent records based on the BBDM Finder Number.
Hope this is helpful!