A linking field is a data field that connects two records. For example, in an automobile claim system, a Smart Object may hold policy holder information, including name, contact information, and the effective dates of the policy. When a policy comes in, a linking field is used to attach that claim to a specific policy-holder record.
The actual content of the field is the internal record ID of the target record--in this case, the ID of the policy-holder record. But to the user, the field appears as summary field that displays information from the target record. To make a selection list, Pulpstream gives them a list of target records they can choose from.