VBR (variable bitrate) encoding lets the bit rate rise and fall with the complexity of the audio, spending more on dense, transient-rich passages and less on simple or quiet ones. For a given average size this yields better quality than CBR, because bits go where they're needed, which is why Opus and AAC use it by default and why it dominates file-based and on-demand audio. The downside is unpredictability: peak rates can spike, sizes vary, and it is harder to fit into a strictly rate-limited channel — so constrained-VBR and CBR modes exist for streaming and broadcast where steady bandwidth matters more than squeezing out the last bit of efficiency.
