This KBA explains which Generic Objects can be used in Business Rules for Offer Approvals.
SAP SuccessFactors Recruiting Management
Standard and custom Generic Objects are supported in Offer Approvals. Custom Generic Objects are editable as long as they are made reportable in Provisioning. Is it possible to set the values in Business Rules for custom Generic Objects, but not for standard Generic Objects, as they are not editable in the Offer Approval. Only editable fields in the Offer approval will be available in Business Rules in “Then” statement, except Location which is a Foundation object. This is true for any fields of the Offer Approval and not specific to Generic Objects.
2411448 - Permission to Edit Offer Approval fields - Recruiting Management
Generic Objects, Business Rule, Offer Approval, Legal Entity, Business Unit, Division, Department, Cost Center, Position, Location, Foundation Object, If, Then. , KBA , LOD-SF-RCM-RUL , Recruiting Rule Issues with MDF Platform , How To