What is the difference between "Constant Bit Rate" and "Constant Quality"?

Constant Quality guarantees the quality of video content. Constant Bit Rate guarantees the bit rate. You can control the file size better by using Constant Bit Rate. If you want to edit the video, and often use the random access (fast forward / rewind) capabilities, Constant Bit Rate is recommended. If you wish to produce a movie for viewing and normal playback, please select Constant Quality. Please note that with higher bitrate, more hard disk space is required.