[Rick Hughes]".. but what does the '12' stand for ?"
It's the number of bits per pixel at an 8 bit color depth. In your case 12 bits per pixel (8 bit YUV with 4:2:0 chroma subsampling, a.k.a. YV12). YUV 4:4:4 would be 24bit, 4:2:2 is 16bit, and 4:2:0 is 12bit per pixel.