Could any of you enlighten me about how to use this variable? And while I am at it (this is more of a general question), is there a way in foobar to use math operations on a variable number of items in the playlist? For example, can I calculate the remaining playback time of a playlist (from a given point)?

Thanks

you can set these variables by going to the preferences -> Tools -> CWB Hooks
they show the defined variables of the next track in the playlist.