I quote from a handy guide I got hold of, dated April 2006.
1. New subsistence package comes into effect today (Apr 06).
2. Day subsistence; up to 21 pounds per day, replacing "Essential Expenses" and "Daily Rates of Subsistence Allowance". Person must be travelling for over 5 hours continuously where service food is not available. Keep receipts, retain for 12 months.
3. Incidental Expenses; up to 5 pounds per day, replacing Personnel Incidental Expenses and Mess Incidental Expenses. Covers expenses incurred when staying overnight in a hotel of temporary service accom. Limited to laundry/dry cleaning, a three minute phone call and a newspaper. Keep receipts for 12 months.
4. Night Subsistence; up to 53 pounds per day if not booked via CHBS (Central Hotel Booking Service), replaces Night Rates of Subsistence Allowance. Payable when staying in a hotel for service reasons. Day subsistence claimed as above for lunch/evening meal costs. Non-availability of Service Accom chit required, hotels normally booked via UPO. Keep receipts for 12 months.
Hope this clears things up a bit.