How does 32BPP AVI work?

[Videogamer555]

It is identified in VLC as RV32 (not sure what RV means) but it is supposed to be RGBX, in other words 24BPP with an unused channel (usually set to 0x00). However the video converter XMediaRecode outputs an actual value near the value of the other color channels for the X channel. And Windows Media Player has no problem playing the file, but there's sure a problem when trying to load frames from it in a raw image loader like GIMP, because it uses RGBA where the final channel is alpha so this makes stuff transparent that isn't supposed to be.

However I may be COMPLETELY missunderstanding what that last channel is for. Maybe the 4th channel IS suppoesed to be used for something, Please let me know what it's supposed to be used for. I'm just curious, because I want to know if XMediaRecode is out of spec with regard to the AVI specs, or if it's just my understanding that's wrong.