CRF (constant rate factor) is a rate-control mode that holds a chosen quality level roughly steady and lets the bitrate vary with scene difficulty, which is exactly the behaviour a quality target wants. It is the practical knob in a per-title pipeline: you rarely command a VMAF target directly, you command a CRF (or the related QP, quantization parameter) value that produces it, often after a brute-force analysis predicts which CRF lands each resolution near its target bitrate. Because CRF spreads encodes naturally along the bitrate axis, it is also how you generate the points that make up a convex hull. The catch is that CRF alone places no ceiling on bitrate, so a very hard scene can run away; pairing CRF with a bitrate cap gives capped CRF. The thing to remember on the measurement side is that the setting only predicts the result, while VMAF confirms whether the encode actually hit the target. It sits within the broader family of rate-control modes alongside VBR and CBR.
