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.