A lockbox is a welded sheet-steel housing that bolts or welds to the latch stile and accepts a mortise lock body inside. The housing is typically 4 to 6 mm steel, sized to fit the chosen lock model (Locinox H-Metal, Lokey, etc), with a hole drilled out for the cylinder and lever spindles.
Lockboxes give a clean look (lock body is hidden inside the box rather than bolted on the face), better security (the lock is harder to attack from outside), and weather protection (the box keeps spray off the mechanism).
CAD60's models offer 'add lockbox' as an option whenever a lever-handle or ball-handle lock is selected. The box dimensions are derived from the lock spec and added to the cutting list.