From FAQs:
3. Am I eligible for fixed flight options for airline tickets?

Certain ThankYou Members have the option of redeeming for air travel at fixed point amounts. Currently, ThankYou Members with a Smith Barney Account, Citibank® Account, Expedia® Account or the following Card Accounts are eligible for fixed flight airline tickets at fixed ThankYou Point amounts: Citi PremierPass® Card, Citi PremierPass® Card - Elite Level, Citi PremierPass® Card – Smith Barney Elite Level, Citi ProfessionalSM Card with ThankYou® Network, CitiBusiness PremierPassSM Card, CitiBusiness® Card with ThankYou® Network, Citi® ChairmanSM Card and Citi Chairman® American Express® Card. This list is subject to change without notice. Fixed flight option Travel Rewards are only available when you call the ThankYou Service Center. Agents will work to offer you the best deal on available flights – you are not required to select the fixed flight option.