Desktop software is a one time purchase. It installs on your computer, so you access it there, typically a work computer or laptop. It is also sold in a modular format (testing, nutrition, exercise and scheduling). For example if you only want fitness assessment you only have to buy that part. You can print or email exercise programs, meal plans, test reports and so on.
But, since it is not online, you obviously lose some functions like the online client portal, video exercise, messaging, etc.