The Custom Extensions module provides you with a facility to register any custom extensions or feature codes that you have created in a custom script or dialplan and the PBX doesn't otherwise know about them. This allows the Extension Registry to be aware of your own extensions so it can detect conflicts or report back information about your custom extensions to other modules that may make use of the information. Also, to keep you from using that extension number somewhere else in the PBX.
For each custom extension, you can define the following
Define the custom extension number that you want the PBX to be aware of and that it is in use. You are not allowed to use the number for something else.
Give this custom destination a friendly name.
Here you define notes on what this custom extension is used for.
When done, don't forget to click "submit changes" to save and click "apply changes" in the top bar.