Until the late 1960's, the RAF operated 'General' and 'Supplementary' Lists. Those on the General List were graduates of the (then) 3 year cadet course at Cranwell, together with officers selected to be transferred from the Supplementary List because they were thought to 'have the right stuff'.
Supplementary List officers were only guaranteed a career to their 38/16 year point but could be 'assimilated' to serve beyond 38/16. Promotion to squadron leader was usually (but not always) accompanied by a transfer from one list to t'other.
For aircrew, a range of short service commissions existed IIRC; 5, 8 or 12 years. Serving airmen/women under age ?? were commissioned into the Supplementary List but over that age they became 'Branch officers' and their promotion was generally limited. Specialist aircrew came along during the '60s, I think, but before then, if you were aircrew and reached age 47, horrible things would happen to you, like your flying pay went down or you lost the lot and became an air trafficer!!
The separate lists were seen not to meet the requirements of a modern air force and they were merged in circa 1969/70. The 3 year Cranwell course was discontinued in about 1972, the graduate entry scheme was all the rage at this time and it was to graduates that the remaining bits of 'preferment' were given.
Sorry for the history lesson - but what goes around comes around. Would be interested to see the rationale for this latest change.
Old Duffer