@khalidhosain The script is designed to work with a single file, that's how I specified it in the description "This script allows you to edit the metadata of a single Mp3 file". If you select multiple files, the changes you make will be applied to all files.
To edit several files at the same time, respecting the different values of each file, I recommend using Steve's script (link), which I also refer to in the script description.
Regarding the "copyright" field (item.metadata.audio.copyright), I did not include it because the intention of the script is really a quick and simple way to edit the most important metadata, I did not want a huge window with a lot of visual content.
In terms of music, I think the "Composers" and "Album artist" (Music producer) fields are most used, however, these days, when I can, I will make a version for you with those fields added.