The Clarion IDE from SoftVelocity is built upon the open source #develop IDE (v2.1.0.2447). The IDE itself is based around a small Core System with all functionality implemented in one or more addins. Using addins we are able to contribute functionality to almost any part of the development environment!
An addin is defined by an \*.addin XML definition file and can optionally include one or more .Net assembly.
In case you are interested, *.sdaddin packages are simply *.zip files renamed to *.sdaddin. If you are curious feel free to open the file and see what is in there!