A Number ID is simply the second section of a callsign. An example from the air force, HSOTF, is "HSOTF13". In this case, the "HSOTF" section shows that the pilot or ATC is affiliated and actively participating in the HSOTF air force. The second part is the Number ID, in this case being the "13". Normally, there is a hyphen between the two sections of a callsign, but on occasion this changes.