I copied this from AME help
Bitrate Mode or Bitrate Encoding Specifies whether the codec produces a constant bitrate (CBR) or variable bitrate (VBR) in the exported file:
Constant:
Delivers a consistent data rate, with the quality potentially fluctuating from frame to frame depending on the degree of compression required.
Variable contrained:
Delivers more consistent quality, with the degree of compression, and therefore the data rate allowed to fluctuate. Allows the exported file’s data rate to vary without limit, with an option to target an average bitrate.
VBR 1 pass:
Variable bitrate, with the encoder making a single pass through the file from beginning to end. Single-pass encoding takes less time than dual-pass encoding, but doesn’t achieve the same quality in the output.
VBR 2 pass:
Variable bitrate, with the encoder making two passes through the file, from beginning to end, and then from end to beginning. The second pass prolongs the process, but it ensures greater encoding efficiency, and often a higher-quality output.
Note:
When comparing CBR and VBR files of the same content and file size, you can make the following generalizations: A CBR file may play back more reliably over a wider range of systems, because a fixed data rate is less demanding on a media player and computer processor.
However, a VBR file tends to have a higher image quality, because VBR tailors the amount of compression to the image content.
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