Originally Posted by
Often1
It means exactly what the contract says it means, e.g., "it is not guaranteed." That is because Hertz may not be able, on occasion, to fulfill the "guarantee" and you are not entitled to anything if it cannot.
Except that the contract doesn't say that. Hertz' Terms and Conditions for President's Circle say this (and thus the OP's question):
"
Guaranteed Upgrade Offer
A one-car-class upgrade is offered at Hertz participating #1 Club Gold locations in Australia, New Zealand, the U.S., Canada and Europe. A minimum 2-hour advance reservation is required for the upgrade to apply. Reserve the vehicle you wish to pay for and receive a vehicle one-car class above the one you requested. The upgrade offer does not apply to coupon offers."
For Five Star and below, Hertz' conditions say that upgrades are subject to availability. There is no such condition stated for President's Circle, provided the advance reservation timeframe is satisfied.