OpenSim.Region.ScriptEngine

ScriptEngine
A ScriptEngine is a .Net Assembly (.dll) plugin that is loaded at server startup.

It can be anything from customized world-scripts such as ALife or environment control to scripts support.

Review OpenSim.Region.ScriptEngine.Common to see how the common logic works and can be used to extend ScriptEngine.

Here is more specific information on XEngine. Also, see ScriptEngines.