This is something new with FileMaker Server 13. A normal Annual Volume License (AVLA) charges you per user machine, plus the FileMaker Server and plus concurrent licenses for FileMaker Go (mobile) and WebDirect connections.
An Annual Site License (ASLA) charges you per employee (total head count) and for that you get unlimited Pro (client), Advanced (us), Server licenses and unlimited Go and WebDirect concurrent connections.
It is significantly cheaper overall providing that your total users needing access FileMaker is close to (or exceeds*) your total staff count.
Annual Site License Agreements
FileMaker Pro 15 is the newest version of simply powerful software used to create custom apps that work seamlessly across iPad, iPhone, Windows, Mac, and the web. Get more time-saving …
Last of the Line
FileMaker recently released a 14.0.6 patch. Nominally it merely installs a new PDF engine and addresses bug fixes. FileMaker does not often patches after a new version has come out, …
If the pace of change in the FileMaker world is getting the better of you, you might like to have a look at this comparison chart from FileMaker.