According to the ServiceNow documentation1, the Business Unit field on a Project or Demand form is a reference field that displays values from the cmn_department table. A reference field stores a link to a record on another table, rather than storing the data directly in the field2.
1:
https://docs.servicenow.com/bundle/vancouver-it-business-management/page/product/project-management/refer
https://docs.servicenow.com/bundle/vancouver-platform-administration/page/administer/form-administration/con