Wendersnaven.com (a resource for nwn2 builders)

The .sef files are special effects files for NWN2. Most of the ones that come with the game are located in C:\Program Files\Atari\Neverwinter Nights 2\Data\NWN2_VFX.zip

From the official documentation:

"Visual effects are composed of effect files, which are individual components of an effect (a particle system, a trail, a billboard, etc.), and SEF files, which point to the effect files and organize them into a more complex visual effect."

As I understand it, .sef files are what you create when you save a visual effect from the visual effects editor.