Mark X started as a purely experimental device operating at frequencies above 1 GHz, the name refers to "experimental", not "number 10". As development continued it was decided to introduce an encoding system known as the "Selective Identification Feature", or SIF. SIF allowed the return signal to contain up to 12 pulses, representing four octal digits of 3 bits each. Depending on the timing of the interrogation signal, SIF would respond in several ways. Mode 1 indicated the type of aircraft or its mission (cargo or bomber, for instance) while Mode 2 returned a tail code.