Here is the verbage from the cert:

50% Discount
Valid Rates: A minimum of the full published retail standard guest room (non-discounted) rate.

Stay Requirement: One certificate valid only for two (2) consecutive nights of a stay of any length. Must include Fri or Sat night stay at Hilton and Libertel Hotels.

Combinable: Up to seven certs for a 14 night stay. Not valid with any other offer or discount.

Avail: May not be avail for all rooms, dates, or hotel properties. # of rooms is limited. Intl properties may be subject to govt approvals, regulations, restrictions, and taxes.

Excluded: Discount does not apply to food and bev, incidentals, svc charges, gratuities, and taxes.

Refunds: No refunds once stay has commenced. Cert has no cash value.

Miles: Miles are not earned when using cert.

Frequent Guest Programs: Credit may not be earned in some programs. Call hotel for details.

Redemption: Reservations must be made in advance. You must advise agent that you are using a Mileage Plus Hotel and Resort Certificate. Original cert must be surrendered at check-in.

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I've only found these certs useful a couple of times. Obviously, the more expensive the rooms, the better "discount" you get. Have fun.