A variant is a specific version of a design.

As PCBs are manufactured for distribution on a global scale, some designs may require modifications depending on their target markets. For example, power supply requirements for the European market differ from those of North America. These variations may call for the use of different components.

You may want to produce a single PCB which would meet requirements for both the North American and the European versions. In this case, the board itself must contain the traces as well as package for both variations of the design. The PCB would then be populated with components according to the target market of the device.