BirdieSync has a "per user" license. So you can use BirdieSync with several machines as long as it is for the same user. So for instance you can synchronize your mobile device with your laptop and your desktop, or with your home and work PC.
Note that ActiveSync only allows you to have two partnerships registered on your mobile. So you can only synchronize your mobile with 2 desktops. If you want to synchronize with a third desktop, you'll have to delete a partnership on your mobile.