The purpose of numbers in the range of three to nine is not defined by the IATA, but they can be used by each carrier for their own specific needs. The first digit is commonly used as a million indicator for the normal six-digit tag number.IATA Passenger Services Conference Resolutions Manual (PSCRM), 33rd Edition, Resolution 740, Secrion 5.1.2; however, this source does not reference how airlines commonly use that digit. It only states that values three through nine are for "interline and on-line use".
