Bitrate Range: specify the minimum and maximum bitrates for your MP3s.

never fall below min. VBR: enable this if you don't want Lame to use a bitrate below your specified minimum. (Note: Lame is pretty smart in selecting good bitrates, so don't limit it. Remember that silence is silent with 8 kBit and with 320 kBit, too.

no Xing tags: VBR MP3s usually have a header identifying them as VBRs. This header contains, by the way, the word "XING" and a table of offsets which makes it easy for players to seek within the file (allowing the position slider of your player to work correctly).