VBR and ABR, Minimum and Maximum Bitrates
For VBR and ABR you can specify a minimum and a maximum bitrate allowed. For maximum, Auto or 320 is recommended. This allows the encoder to use the highest bitrates for more demanding portions of the music. The highest bitrates are only used as needed, not constantly.
For minimum, Auto selects the appropriate bitrate or you may specify a minimum. You may also check the box for Enforce min., which will strictly enforce the minimum bitrate selected. (Good for players that do not support low bitrates). If unchecked, silence will encode below the minimum.