Cty.dat, or sometimes wl_cty.dat, also known as a “country file.” This file contains the lookup information that translates prefixes of callsigns into country and/or zone multipliers. Software uses this file, say, to tell you that N9RV counts as a W, and is in CQ zone 4 (or ITU zone 6). Since worldwide prefixes are frequently in a state of flux, an up-to-date country file is always desirable. More recent country files also contain data that may help determine a station’s zone. Such information can be inaccurate and in all cases operators should gather such information from the contest exchanges themselves.
An Amateur Radio Blog by DX Engineering 
