Variable bitrate encoding lets the encoder spend data where the picture needs it. Complex, fast-moving or detailed scenes get more bits; static or flat ones get fewer. The average over the file is what you see quoted as "the bitrate".
Nearly all file-based encoding is VBR, including both of the modes you normally choose between:
- CRF is VBR with a quality target and no size constraint. Quality is held steady and the file lands where it lands.
- Target-size two-pass is VBR with a size constraint. Total bits are fixed and distributed by difficulty.
The reason this matters when reading specifications is that a single bitrate number describes a VBR file only loosely. A clip quoted at 5,000 kbps may peak at three times that on its worst second. Platforms that enforce a maximum bitrate are usually enforcing a peak, not an average, and a file that satisfies the average can still be rejected.
That is what maxrate and bufsize settings control: a ceiling on the peak
and how much the encoder may borrow against it. Constrained VBR — variable
within a cap — is the usual compromise for delivery, and what platform upload
specifications are generally asking for when they publish a recommended
bitrate alongside a maximum.