This feature is based on matching text by using Regular Expression (RE) [see this appendix].
To the "Regular Expression formula:" field, you have to write a regular expression.
After you "Check" the expression, you can select, what matches are what field. If you have all selected, you can click continue and program will do its own work. Every filename is scanned and the matches are inserted into the tag field.
If this checkbox checked is, program will ignore the ".mp3" extension of file. There is no meaningful reason to uncheck this checkbox. But this is option is here.
If this checkbox checked is, program will use for RE matching the whole path (usefull if you have folder named like i.e. the album).
In this box you will see filepart matches (after testing RE) and also you will be able to select which part of file is what ID3 tag field.