When using VBR, the encoder buffer is allowed to underflow (be empty), and the maximum bitrate, which is the transmission bitrate used for the buffering model, can be higher than the target bitrate. So VBR relaxes the buffering constraints and allows to decrease the bitrate for simple content and can improve quality by allowing more bits on complex frames. VBR mode constrains the bitrate with a specified maximum while keeping it on the target bit rate where possible. VBR is like CBR in that it avoids buffer underflow by increasing the QP. However, the target bit rate can be exceeded up to the maximum bit rate. So, the QP must be increased by a smaller factor. A buffer overflow results in an unchanged QP and a lower bit rate.
Both CBR and VBR use frame-level statistics from the hardware to update the initial QP for the next frame (rate control can be combined with QP control that can adjust the QP at block level for improving subjective quality).
Low-latency rate control (hardware rate control, see property alignment) computes the QP at block level to reach a target bitstream size for each frame in an accurate way, and is especially useful for the support of low-latency pipelines.