The following is from a knowledgable source: As far the numbering convention is concerned, the 11, 21, etc. is usually either based on the “go” of the day OR the the flight within a “go”. To explain, a “go” may usally consist of 12 jets. Of those 12, there may be three formations. Therefore, we normally assign the numbers of those formations as a sequence, . 11-14, 21-24, and 31-34. Where there are multiple flying squadrons at a base, they assign each squadron a series of 3 numbers, 0-8. Therefore, the same number suffix should not be flying with more than one formation at a time. This is also helps controls and pilots recognize their callsigns on the radio, especially if the radio transmission is clipped. For example, if all that is understood was “(radio static) 71, descend and maintain 3 thousand”, one would still know that the message was for callsign 71 and not callsign 51. The “91” callsign suffix is usually reserved for “checkride” sorties.