Adding Approval Status to Objects
After the picklists have been set for objects, it's time to set the object picklist field so that Approval Status is available on the object record.
To add a picklist field for approvals to objects
Lookup Relationship with Approval Request Object
To have visibility on approval requests that are associated with the custom object in a related list or in Approval Center, you must create a lookup relationship between the approval request object and your custom object.