The DCS240 offers 400 locomotive slots and can support up to 400 throttles. These slots can be broken down in to standard slots and expanded slots. There are 120 standard slots in the DCS240, these slots are accessible using any Digitrax throttle. There are an additional 280 expanded slots available to DT402 R2 (tethered and Duplex) and later throttles. The DCS240 identifies the throttle type being used and assigns and adjusts the slots accordingly. Expanded capability throttles are given the Expanded slots first, leaving the standard slots available for legacy throttles. All this is done in the background and will not affect any running trains.
Practically you will likely not be using 400 throttles in your operations. However, when consisting locomotives together loco slots can quickly be used up. This is where expanded slots become very useful. When building a consist using an expanded throttle the DCS240 will initially store all the locos in available expanded slots. In a consist the throttle only needs access to the TOP loco of the consist to control the consist. When selecting a consist using a legacy throttle only the TOP address is moved to a standard slot, the remainder of the locomotives remain in the expanded slots and they are all linked inside the DCS240. In this way as many standard slots are left available as possible.