The Ranger library was built to allow developers to validate against their own parameters and call back to their licensing server however they wanted to. It allows for hard and soft license check failures, preventing your apps from being stranded in the wild in the event of your license server going offline.
The best place to start if you need help with a specific product is to contact the developer. All WHMCS Marketplace developers have both a website and support URL listed.
Build, License & Distribute Your Own Apps using WHMCS
This addon allows your to add html / php to the header / footer in the admin area / admin login page / clients area
Simple and easy way to debug your current page (defined constants, request variables, queries)
This module allows you to alter the language strings used throughout WHMCS.
WHMCS addon for expose a RESTful API