user_fld is a new feature introduced in ArcGIS 10. It's quite useful to put additional fields such as a parcel ID in that field to avoid extra query round trip for single house style locator. It's working very well in ArcGIS 10.
However, after SP1 (then SP2 & 3), it seems that functionality is having trouble. For locators created with ArcGIS 10 but published to SP1-3 (basically after applying for service packs to ArcGIS server), it is still available. The problem is for new locators created with ArcGIS 10 Sp1 and later. Although the create tool still asks for that field, the resulting locator no longer advertise that field in "candidate fields", even after the "writeAdditionalOutputFields" in the .loc.xml is changed to "true" manually.
Anyone else is having the same problem?