Wikipedia
A modex is a number that is part of the Aircraft Visual Identification System. It usually consists of two or three numbers used by the Department of the Navy, U.S. Navy and U.S. Marine Corps aircraft that identify a squadron's mission and a specific aircraft within a squadron. These numbers are painted conspicuously on the aircraft's nose, or on helicopters; sometimes on the aft portion of helicopters fuselage or forward portion of the empennage. Modexes are also painted less conspicuously on other aircraft areas (i.e., fin tip, flaps, etc.). Shore-based aviation units will use either two-digit or three-digit modexes, while carrier-based units always use three digits.
In a Carrier air wing (CVW), the first digit of a modex number indicates the squadron to which the individual aircraft is assigned in the CVW. Modex numbers in the 1xx and 2xx series are assigned to the CVW's Super Hornet Strike Fighter Squadrons (VFA), which fly either the F/A-18E or F/A-18F Super Hornet. Modex numbers in the 3xx and 4xx series are assigned to the CVW's remaining two VFA squadrons (or, if a Marine Corps squadron is assigned, VMFA), which fly the F/A-18C Hornet. As more USN F/A-18C Hornet squadrons transition to the F/A-18E or F Super Hornet those 3xx and 4xx modexes may also belong to VFA squadrons flying the Super Hornet. All CVWs have four VFA squadrons or three VFA and one VMFA. Some are all Super Hornet E or F squadrons and some are a mix of Hornet C model squadrons and Super Hornet E or F model squadrons.
5xx side numbers are assigned to the Electronic Attack (VAQ) squadron flying the EA-18G Growler.
Modex numbers with the first digit of 6 are assigned to the Airborne Early Warning (VAW) squadron (600-609) flying the E-2C (currently transitioning to the E-2D) Hawkeye, and the Helicopter Sea Combat (HSC) squadron (610 and higher) flying the MH-60S Seahawk helicopter.
7xx modexes are assigned to the Helicopter Maritime Strike (HSM) squadron flying the MH-60R Seahawk helicopter.
The last two digits identify an individual aircraft within a squadron with the aircraft numbered sequentially from x00 (or 610 in the case of the HSC squadron) up to the number of aircraft in that squadron with the number x13 often skipped. Some squadrons may skip numbers other than x13 if past mishaps resulted in the loss of an aircraft or crew with that number. Each squadron will typically designate the aircraft with the modex number of x00 (i.e. 100, 200, etc.) as the CVW commander's (CAG's) aircraft (called the CAG bird) and will paint its tail with squadron colors rather than with the subdued grey tone colors of the rest of the squadron's aircraft. Aircraft with a modex of xx1 (i.e. 101, 201, 701 etc.) generally carry the name of that squadron's commanding officer and those with a modex of xx2 (e.g., 102, 202, 702, etc.) generally carry the name of the squadron's executive officer.
Marine aircraft are typically shore-based and use a two-digit modex. If USMC aircraft are assigned to a carrier air wing they employ the three-digit system used by the rest of the carrier airwing.
Land-based naval transport aircraft (VR) may use three-digit modex numbers but use of the last three digits of the aircraft's bureau number (BUNO) is also not uncommon. Carrier onboard delivery aircraft (VRC) also used BUNO-derived side numbers, but occasionally use the aircraft carrier's number (for example, 68 for an aircraft assigned to USS Nimitz: CVN-68).
Land-based patrol aircraft (VP) previously used a two-digit modex number until the mid-1990s when they transitioned to three digit modexes using the last three digits of the aircraft's Bureau Number (BuNo).