Average bitrate is the mean data rate a video stream produces over time. Where constant bitrate pads a quiet scene to a fixed rate, average bitrate lets that scene use fewer bits and a busy one use more, so storage can be estimated from the average while the network still carries short bursts.
Bitrate settings change what a camera does with an empty hallway. Average bitrate is the setting that lets the camera spend data where the picture actually needs it.
Average and constant compared
Constant bitrate guarantees a fixed rate, which makes network planning simple but wastes space on a still corridor. Average bitrate targets a mean over a period, so a busy car park costs more and a quiet stairwell costs less. Both depend on the recorder and the network tolerating the peak.
Sizing storage and viewing
Use the average to estimate daily storage, then check the peak against the link and the recorder’s write capability. Remote viewers feel the peak, not the average. Where footage may have to settle a dispute, keep the setting generous enough that motion is not starved of bits.