I know that F=fighter, A=attack, F/A=fighter/attack, B=bomber. I can not find any information on what YF stands for. This type of aircraft is usually an experimental aircraft, but why do modern aircraft use X instead of YF?
The 'Y' in YF stands for prototype according to the Tri-Service aircraft designation system. The 'F' stands for fighter, so YF stands for prototype-Fighter. These aircraft are operated by the US Department of Defense (USAF or USN).
The 'X' series is the name given for strictly experimental aircraft; i.e. they are not prototypes.