The Sage is one of the endgame jobs of Final Fantasy III, alongside the Sage. How it is accessed varies between versions: In the original games, it was awarded with the Elder Staff in Eureka. In the 3D remake, it was moved to the Earth Crystal, placing it with the rest of the final batch of jobs.
The Sage is a textbook Jack of all trades in the original Famicom version: It has stats on par with the other mages in the endgame as well as the highest MP growths. This means that there's no real reason to use any of the Earth Crystal magic jobs once you go to Eureka and get the Elder Staff (technically, you don't have to but outside of the challenge aspect, there is no reason not to). The only downside of a sage is having to determine what to fill the three magic slots of each tier with.
One of the major aspects of the remake's job balance was to ensure that most jobs were viable. While this largely resulted in buffs for each job, the Sage got heavily nerfed into a Master of None: Its MP growths were made worse while its stats pale in comparison to the specialists. While it still retains its utility, it's not as overpowering as it used to be.
Despite the Pixel Remaster being based on the Famicom version supposedly, the Sage retains the lowered Mind and Intelligence levels, although its agility have been buffed from 3D (albeit, not to the same extent it was on Famicom).
The Sage can use magic from every school and at every tier. It still retains the 3 spells per level limit though, meaning that consideration must be given to each level.
In the Famicom and Pixel Remaster versions, the Sage strictly uses the Summoner versions of the summoned beasts. In the 3D versions, it can only use the Evoker's spells.
Due to the nature of the Sage in the remake, the guide recommends using it as a support for one of the specialized jobs. The flexibility of the Sage means that it can multitask in a pinch. However, it should still be geared towards the role in the party that it will be playing. In regards to summons, priority should be given to ones where both of the effects are useful, as well as spells with particularly strong effects.