The 'owner' field

JabRef can optionally mark all new entries added or imported to a library with your username.

You can disable or enable this feature by entering Options → Preferences → General, and selecting/deselecting the line 'mark new entries with owner name'. You can also change the name used to mark entries. By default, your user name is used. Finally, if an entry with an existing field 'owner' is imported, the field is updated with your name if 'Overwrite' is checked.

The owner name is added in a field called 'owner', which by default is visible in the General fields tab in the entry editor.