With the release of trixbox Pro v5.2 there are some really significant changes to how some of the features now work. The biggest change is with the users and extensions, which also affects licensing. With version v5.2 extensions and users are now separated and a user can have up to four extensions assigned to him/her. Where licenses where based on extensions before, now they are based on users. If you had to buy extra licenses so some users could have both an office phone and a remote phone, with version 5.2 this would only require a single license.
Check out the video for a more complete explanation and live demo of how this feature has changed.