Could someone please be kind enough to tell me where the source of this information is? The main website says nothing about prerequisites for the SMN or SCH classes...
SMN and SCH are jobs. You have to level Classes - Arcanist to 30 and it's main sub class to 15 (CNJ is assumed to be). Ones you unlock the jobs your SCH,SMN, ARN will gain exp together regardless of which one you are physically using. Its impossible to use SMN or SCH without having leveled ARN first.
Yeah - I believe it has been datamined, but I don't have proof, no - but, a good guide is, through the abilities lists at http://xivdb.com
, check the "equippable classes" list at the bottom of each ability. The Jobs that can use the ability are listed at the end. The cross-classable CNJ & THM abilities can be used by both SCH & SMN. There may be something we don't expect with having 2 "sub" classes for a dual-job class like ACN, but it's a start.
As Death said - Arcanist is the base class
. Summoner & Scholar are specialised Jobs
for this class. Think of Jobs as "stances" for a base class - it restricts your cross-classable abilities, but grants new Job-specific abilities that help specialise your role. SMN enhances the DPS capabilites of the class - a new damage-based summon (Ifrit) in addition to replacing Carby with better versions (Garuda - magic dmg, Titan - tank) and a couple of new spells. SCH turns you full healer/support - 2 fairies that focus on healing & support, plus some more healing spells for the PC.