It rename files with given padding mask. Here total length of new file name will never be more than given mask. For example you want to rename file names like this.
If Mask is 0000 then,
Now Mac file name could have all these characters except : (Colon)
So if you are someone who frequently exchange files to windows users this feature is just for you.
It will just replace invalid characters with the character(s) of your choice. If you just want to remove these invalid characters then do not type anything in text box.