It is described in the A3 manual in detail.http://server.conitec.net/down/a3p.zip
But you don't have a format like WMB. It's a mixture of script definitions and a very simple WMP format. Even when converting everything to Meshes (like I do with A3 Tools) there is no 100% solution unless you write a full WDL parser which is clever enough to detect moveable level parts.
Moveable parts can easily destroy any mesh triangulation and may need special treatment depending on what exactly is being moved. That's the point where I did not develop A3 Tools any further.
The problem is A3 does not use meshes - it's a Doom-style Raycaster. Doom had a predefined list of what could be done, making it easier to detect special cases. In A3 it's all done in WDL.