Force characterizes this designation system as "MDS", while the Navy, and Marine Corps refer to it as Type/Model/Series (T/M/S). Status prefix These optional prefixes are attached to aircraft not conducting normal operations, such as research, testing and development. The prefixes are: e: Digitally developed G: Permanently grounded J: Special test, temporary N: Special test, permanent X: Experimental Y: Prototype Z: Planning A temporary special test means the aircraft is intended to return to normal service after the tests are completed, while permanent special test aircraft are not. The Planning code is no longer used but was meant to designate aircraft "on the drawing board". For example, using this system an airframe such as the F-13 could have initially been designated as ZF-13 during the design phase, possibly XF-13 if experimental testing was required before building a prototype, the YF-13; the final production model would simply be designated F-13 (with the first production variant being the F-13A). Continuing the example, some F-13s during their service life may have been used for testing modifications or researching new designs and designated JF-13 or NF-13; finally after many years of service, the airframe would be permanently grounded due to safety or economic reasons as GF-13. Modified mission Aircraft which are modified after manufacture or even built for a different mission to the standard airframe of a particular design are assigned a modified mission code. They are: A: Attack C: Cargo (i.e., transport) D: Drone director E: Electronic warfare F: Fighter H: Search and rescue, MEDEVAC K: Tanker L: Modified for cold weather operations M: Multi-mission (i.e., Special Operations) O: Observation P: Maritime patrol Q: Unmanned aerial vehicle R: Reconnaissance S: Anti-submarine warfare T: Trainer U: Utility V: VIP transport W: Weather reconnaissance The multi-mission and utility missions could be considered the same thing; however they are applied to multipurpose aircraft conducting certain categories of mission. M-aircraft conduct combat or special operations while U-aircraft conduct combat support missions, such as transport (e.g., UH-60) and electronic warfare (e.g., MC-12). Historically, the vast majority of U.S. Coast Guard air assets included the H-code (e.g., HH-60 Jayhawk or HC-130 Hercules). In the 21st century, the Coast Guard has used the multi-mission designation for their armed rescue helicopters (MH-60 Jayhawk or MH-65 Dolphin). Basic mission All aircraft are to be assigned a basic mission code. In some cases, the basic mission code is replaced by one of the modified