Multiplayer rules are defined by server-side scripts written in Lua and are highly flexible. The TES3MP code consists of a client and a server.
As such, it is not a mod in the traditional sense, but rather an alternative game engine for The Elder Scrolls III: Morrowind with an added multiplayer component. TES3MP is a fork of OpenMW that provides multiplayer functionality.
The main logo of TES3MP, which is derived from the OpenMW logo Home Page