Short delivered means fewer items were delivered than booked. The driver records it in the app (items delivered, the reason and photos), or the office does: press Record short delivery on a delivered job, or enter the Items delivered in Mark delivered. The job stays Delivered with an amber "Short delivered: x of y items" panel showing the reason and photos.
The customer sees it in the portal (with the reason and photos), on the tracking page and on the printed proof of delivery. Press Change on the panel to correct it; entering the number booked removes it.