Each field has a checkbox beside it to indicate if the tag will be written to the selected files when the OK button is pushed. If only one file is selected, the checkboxes are not visible and all tags are written to the file. If more than one file was selected, only the attributes that are common to all files are selected by default. You can enable the unshared field by putting a check in the check box.
The text fields on this dialog are mostly free-form, so enter whatever tag information you like.
Note: Re-writing tags may take a while, depending on the size of the original files.