The "Prefix" is the hierarchical name(s) shown as part of the location for a request.  For example, if "Suite 100" is added to the location tree, under "Exchange Bldg", then the prefix is "Exchange Bldg".  On a request for "Suite 100", the location field would appear as "Exchange Bldg, Suite 100".  The "Prefix" helps you identify which "Suite 100" needs service.

Tap here for information on editing a location "Prefix" and here for an equipment "Prefix".