Quality-based VBR. With quality-based VBR encoding, you specify a desired quality level (from 0 to 100). Then, during encoding, the bit rate fluctuates according to the complexity of the streama higher bit rate is used for intense detail or high motion, and a lower bit rate is used for simpler content. The advantage of quality-based VBR encoding is that quality remains consistent across all streams for which you specify the same quality setting. The disadvantage is that you cannot predict the file size or bandwidth requirements of the encoded content before encoding. Quality-based VBR encoding uses one-pass encoding.
If you are encoding audio content with this encoding mode, you have the option of using a lossless format. Lossless encoding produces superior quality results, while also providing some compression of the data.