Some information from theencoderdeveloper for your reference,
In IPP H.264 there are two BRC modes indeed: the so-called CBR and VBR, they mainly differ in the internal constants which regulate the averaging periods the speed of reaction of the BRC algo to the deviation from the target average bitrate. CBR provides faster reaction, hence better bitrate keeping, at the expense of quality.
The BRC mode can be set in the umc_video_enc_con application via the parameter file, 1 corresponding to CBR, 2 to VBR, 0 to constant QP mode.
And some informationinhttp://en.wikipedia.org/wiki/H.264/MPEG-4_AVC