I see you are talking about "Related Pages". The field is empty because this control replicates Related pages functionality, It just lets you do the same thing on the form tab that you could do on properties/related pages tab. All data is stored in CMS_RelationshipName and CMS_Relationship tables. You can use Document Query for that
// nodeGuid - is the Node Guid of your document
// Get documents related to specified document
var documents = DocumentHelper.GetDocuments("CMS.MenuItem")
.OnSite("CorporateSite")
.InRelationWith(nodeGuid, "IsRelatedTo");