Some Explanations concerning the new MP3 tab :
Some functions under BeSweetGUI are outdated. This is the case for MP3 part. We know all that BeSweet uses one of the best mp3 encoder more exactly LAME. But some people as Gabriel Bouvigne by example, one of the LAME developpers, have suggested me to involve the mp3 tab to facilitate and enhance comprehension for users. That's why I follow this recommandations and some new things are appeared :
With the latest LAME releases (3.95.xx and sup), the developpers want to simplify as much as possible the using of this encoder under GUI. That's why, now we can have the choice between "Target Bitrate" and "Target Quality". This is the most important thing to know actually.
The "Target Bitrate" refers to ABR/CBR Mode. I hope everybody know what is it. So, as soon as you choose it, you can move the Bitrate slider to fix a target following your bitrate. The range is very large as you can see. By default, the bitrate is 128 kbps.
To access at CBR Mode, you must check the "Restrict Encoder to Constant Bitrate" case. In this way, you force LAME to have a constant bitrate. The slider of ABR Mode enables to choose the bitrate.
Now the second choice : "Target Quality". This Mode is equivalent to the famous VBR Mode. So, if you check this, you can choose a certain quality level (between 10 and 100). If you are used to transcode your files into Ogg Vorbis with the Q-Mode, you know it's the same idea. I think it's more easier to understand a level quality than to choose a bitrate level.
One important thing you must to know : more this quality level is low, worse the result final is. A good point could be to choose a level between 50-80. It's more safer. By default, it's a value of 80 which is taken. For people who know LAME command, this quality level corresponds to -V 2.
With this "Target Quality", we can have also the choice between the Fast Variable Bitrate Mode and the Standard Mode. By default, it's Standard Mode which is chosen. For information, this Mode refers to --vbr-old LAME command whereas Fast Mode is --vbr-new.
The last part concerns the possibility to active the Mono Encoding. The "Other Settings" is available but *only* for advanced users. In the most cases, the best for you is to play with "Target Bitrate" or "Target Quality".
Due to the fact BeSweet doesn't recognize some single LAME commands, I was forced to use some old arguments.That's why I put here a table to have some equivalences. I hope this help you.

Quality Levels | Lame Switch | BeLight Switch * | Bitrate Equivalence | Presets
Quality = 10 | -V 9 | -m j -v --vbr-old -V 9 | 64 kbps |
Quality = 20 | -V 8 | -m j -v --vbr-old -V 8 | 80 kbps |
Quality = 30 | -V 7 | -m j -v --vbr-old -V 7 | 96 kbps |
Quality = 40 | -V 6 | -m j -v --vbr-old -V 6 | 112 kbps |
Quality = 50 | -V 5 | -m j -v --vbr-old -V 5 | 128 kbps |
Quality = 60 | -V 4 | -m j -v --vbr-old -V 4 | 166 kbps | --(alt)-preset medium
Quality = 70 | -V 3 | -m j -v --vbr-old -V 3 | 176 kbps |
Quality = 80 | -V 2 | -m j -v --vbr-old -V 2 | 192 kbps | --(alt)-preset standard
Quality = 90 | -V 1 | -m j -v --vbr-old -V 1 | 224 kbps |
Quality = 100 | -V 0 | -m j -v --vbr-old -V 0 | 256 kbps | --(alt)-preset extreme

* : If it's Fast Variable Mode which is chosen, then BeLight uses --vbr-new instead of --vbr-old. With Mono Encoding, BeLight replaces -m j by -m m. For ABR Mode, it's --abr xxx where xxx is the bitrate value. And for CBR, BeLight uses -b xxx command.