WordPress Filter is a comprehensive post filtering & template system. It allows the user to define a set of simple "Catches" (criteria) to be met
A: Simply a comma-separated list just as you would do on the add/edit Post screen. (ex: "2008, photo")
A: A post can have multiple filters run on it, but a specific filter cannot run on a specific post more then once. Depending on community feedback, the ability to let a filter run on a post more then once might be added to a future release.
A: To use this, on any post that you want WordPress Filter to potentially engage on, just define the custom field with whatever name you setup as the Catch, then give it any value (the value "1" works perfectly well).