Sometimes reservations just don't work out. For whatever reason, prospective guests decide to cancel a booking, or not to complete it in full. This is especially common where bookings are made online, where it is so easy to click off a web page and look elsewhere on the Internet for a perhaps better (lower priced) deal.
For a hotel, when a reservation is not made in full, this is regrettable, hence such a booking being known in the Hospitality Industry as a Regret. Hotels large and small should always log all regrets, as this can help them when setting pricing strategies each year. If they can anticipate a certain number of regrets (based on statistics drawn from previous trading years) they can at least take steps to cover this loss in revenue in other ways in the future.
Where a hotel tells a prospective guest that a requested room type is sold out, this is called a Denial. Denials and Regrets are part and parcel of hotel management, and savvy hoteliers and large hotel chain revenue management teams will factor Regrets and Denials into their pricing and revenue management. However, with Regrets it is impossible to make wholly accurate forecasts, as much of it is still left to guesswork once all gathered statistics and other key data is carefully analysed.
Finally, it is important to mention that many hotels suffer numerous Regrets each year, simply because the prospective customer used a different distribution channel (an OTA, a GDS, or a high street booking agency, etc.) at the last minute. They opted for an alternative deal at another hotel advertised through a channel, meaning that there was not direct contact with the hotel they were initially interested in staying at. In most cases, most hotels will be unaware of the interest, but where the prospective guest has at least completed some part of the reservation process (but then abandoned the purchase) this information may still be logged and then registered in hotel records as a Regret.