Both FME Server and FME Desktop require licensing in order to use them. FME has two licensing methods, the method you use will depend on your specific requirements:

Fixed

A node-locked license is for use with FME on a specific computer only. The license cannot be transferred to another computer except by a special request to Safe Software.

Floating

Floating licenses are held on a server and issued to individual users as they start up FME. This is useful for the situation where there are many FME users, but not all using FME at the same time.

All editions of FME Desktop have the same installer, with different functionality being unlocked by different licenses. If you are using FME Server only the floating licenses option is available.