In determining routing for accrual or award travel, "between" indicates that travel may originate on either end of a route. For example, a roundtrip award that allows travel between Mexico and the U.S. means that travel can originate in either the U.S. or Mexico. Conversely, roundtrip travel to Mexico from the U.S. means that travel is restricted and must originate and terminate in the U.S.
Between in Anne Mc Caffrey's "Dragonriders of Pern" series, refers to a dimension through which dragons travel during the process of teleportation, i.e. moving instantaneously from one place to another. If the dragon does not have a clear picture of its destination in mind, it is possible for the dragon and its passengers to become permanently lost in this dimension. Deliberately going between without a destination in mind is how dragons suicide, usually occurring when the rider that they are soul-bonded to dies of illness or injury, or if the dragon itself is mortally injured.