'Decrease BPP' - command available ONLY IN 24BPP MODE. Analogue to 'Decrease CLR', but deletes some colors to reach one of bit-borders (borders are 1,2,4,8,16,32,64,128,256 colors count). If when you save GIF (in 8bpp) - you will enable optimization option - then less BPP is less file size (but and less quality). By 'Decrease BPP' you can decrease 'Bits-Per-Pixel' value of you picture before saving in optimized GIF.