When people say "Fed" boxes they are just talking about boxes they get from the bank of machine wrapped coin. They're really boxes from armored car companies and generally come in two styles (at least as far as I've seen). In my area the more common are boxes with paper wrapped coins (wrapped with NE Strings rolls) and rolls that stand upright. The less common (in my area) are boxes with plastic wrapped coins that are laid on their sides. In my experience banks that have plastic wrapped coins (pennies, nickels, dimes, quarters, small dollars) have half dollar boxes that are half as wide as the first type and twice as tall, coins are stacked two rows high.
I have frequently heard the term "Fed" boxes used at banks as well, or sometimes I hear them say "boxes from our armored car company," "boxes we order", etc.
Personally I prefer getting customer wrapped coin when I can. I have greater success with that. If I could find a bank that would sell me bags off their coin counter I'd try that as it seems to have a similar success rate.
Brian
(PS - at least for non-halves my success rate for customer coin is only marginally better, I just get lots more foreign coins which keeps it interesting, and have a better chance of finding grandpa's stash of funny coins that someone just turned in)