If running on hardware that has pixel shader support greater than that which is to be supported by a product, it's useful to be able to force the pixel version down to a lower level. It allows debugging and testing on that class of hardware without needing to actually install it.

To force the engine to run at a particular version, look at the client/prefs.cs file. Change $pref::Video::forcePixVersion to 1. Then change $pref::Video::forcedPixVersion to whatever you wish to force it to (1.1, 1.4, 2.0, etc). Check the console when the engine has started up to see if it has worked. It should have the lines:

Pix version detected: X
Forced pix version: Y