In planes, a variant is a specific type or model. Usually used to describe prototypes. The JSF has many variants that are all based off of the same design, but are designed for specific objectives. Four of the variants are used to perform different functions. For example: the USMC version has VSTOL capabilities and the USN version has a strengthened structure. The SR-71 has many variants including ones with a pilot and a navigator. Others were designed to be high capability fighters for the USAF.