Don't think I've used logical operators but I knew I'd seen it used somewhere when looking at existing themes.
A text search reveals the theme Redbox (by Michal Santavy ) includes the line
%?and(%if(%St(eq precut), >=, 60),%if(%St(eq precut), <=, 120))<%xd(Ka)|%xd(Kb)>
So presumably that's how it's supposed to be used?