So in this case the EX suffix is just a marketing thing chosen by Boeing. It more or less means "extended." The F-15 is an obsolete aircraft from the point of v

So in this case the EX suffix is just a marketing thing chosen by Boeing. It more or less means "extended." The F-15 is an obsolete aircraft from the point of view of procurement, but Boeing has sold the AF on buying a few more of an updated model to extend the life of the F-15 in the fleet for a few more years via the acquisition of new planes directly as well as reducing hours on existing airframes. There's considerable debate whether this is a sensible decision financially for anyone besides Boeing.