User filter is not used very much, but it can be highly useful in certain cases.

First of all, the user filter list is found on the 'Clip Librairy' window and you can edit them in Options.

If you know C function 'printf' or Pascal 'format', then filter is the argument and the text (see below) is the parameter.
If you don't know function 'printf', then here's how it works:
"printf('<strong>%s</strong>', 'so free')" gives "<strong>so free</strong>"

So, in the 'User data' filter, you provide a string (eg: "<strong>%s</strong>"). In editor there is two ways to apply it:
- Shift+Ctrl+V to give the Windows clipboard text as a parameter,
- Shift+Ctrl+F to give selected text as parameter.

Of course, "<strong>%s</strong>" is basic sample, but "</tr><td>%s</td><tr>" will be more useful.
Now, it's up to you, to discover all the possibilities.