Lookup fields allow you to select a record from one object and populate it on another.  For example, on the Incident form, there is a Lookup field on the Incident object to the Category object.  This allows a user to select a Category from an Incident.
Fig. 1 Incident Layout
The BMC Remedyforce Category object is used as a lookup on multiple objects – Incidents, Tasks, Change Requests, and Problems.  Not every category should be selectable on the Incident form, so BMC added a Lookup Filter to filter out certain Incidents.  See Fig. 2 below.
Fig. 2 List of Categories
On the Category object, a field labeled “Available for Incidents” and a field labeled “Inactive”, both checkboxes, are used within a Lookup Filter to limit the number of Categories that will display when clicking the Category lookup button on the Incident form.  Based on Fig. 2 above, three categories will filter out of the results: App – HR, App – IT, and App – Sales.
Fig. 3 Lookup Filter on the Incident Object
The Category Lookup Filter on the Incident object is Optional.  The categories will be filtered based on the Filter Criteria (only show records WITHOUT a checkmark in the ‘Inactive’ field and records WITH a checkmark in the ‘Available for Incidents’ field) but a hyperlink: “Show all results”, can be clicked to bypass the Lookup Filter.
Fig. 4 Popup – Displayed when the Lookup Filter on the Incident Object is clicked
When searching for “App – IT”, zero results are returned.  If the “Show all results” link is clicked, the “App –IT” category record will then be displayed.
If using a Lookup, it is possible that far too many records are displayed in a popup.  This is especially true if one object is used as a Lookup for multiple other objects as is the case with the BMC Remedyforce Category object.  A Lookup Filter can be employed to limit the results to only the necessary records.
David Judkins, RightStar Systems