This feature enables to enhance and configure GenTle's functionality without need to build it again. You can build a plug-in separately and add it to the GenTle as shared library. Plug-ins are used to produce different kinds of outputs of data you have created. There is only one plug-in defined at this time called SingleFT. This plug-in is able to save family tree created in bitmap file. Generally spoken, plug-in can read data and create output in arbitrary form but plug-in cannot modify GenTle's data. If it does so, data can be damaged and behavior of GenTle is undefined in that case.
If you want to build or create plug-in, read plug-in HOW-TO.