A mask is the file name or file filter (or semicolon delimited group of ones) plus the folder name where to find them.

You can use wildcard characters - asterisks (*) and question marks (?) - in the filter. An asterisk matches any number of characters. A question mark matches a single arbitrary character.

The file matches the mask if and only if it is located in the mask folder and it matches the mask filter.

If you set the Include Subfolders option of the mask, files located in the subfolders of the mask folder and match the filter will also match the mask.