When a unit meets a certain Ki requirement, which is usually 12 Ki, its ATK is multiplied by a number, this is basically what a Super Attack is. Different units receive different multipliers and part of it is shown in the Super Attack description: this is what the "causes extreme/supreme/immense damage" means, but it's not the only thing that determines how big of a boost a unit's ATK gets. The second part, which is actually the first one to be factored in, is the "Ki multiplier", which is basically a boost that a unit receives the more Ki it gets. To make a more practical example Unparalleled Super Saiyan Super Saiyan 4 Vegeta has a 150% 12 Ki multiplier and an "immense damage" Super Attack multiplier, that equates to a 505% ATK boost; when SS4 Vegeta reaches 12 Ki, his ATK stat is multiplied by the Ki multiplier first and then by the Super Attack one and his "Super Attack animation" starts.
Now I said that SS4 Vegeta's "immense damage" equates to a 505% ATK boost, but it's not completely true because 505% is the boost he would get at "MAX Super Attack Level"; SS4 Vegeta's "natural" SA multiplier is actually 280%. As you can see that's a very big difference between the two, so how to do I get SS4 Vegeta's "Super Attack Level" to "MAX"? To increase a unit's "Super Attack Level" there are basically two ways: you either use a Mysterious Ritual Elder Kai / [Thrilling Christmas Eve] Master Roshi (Santa) / Grand Kai (INT), special characters that can raise the Super Attack Level of one of your units by 1 by using them as training partner, or you utilize a character that shares the same name as that unit.
Farming the Super Attack Level (SA)
"Farming the Super Attack" means utilizing any unit that is obtainable in-game, or is at least very easy to acquire, to raise the SA of another unit, it can be a very short process with little to no investment or it can be a very long grind that requires a lot of medals, zenie, training items and time. This method is usually preferred over simply using one of the special characters I mentioned in the other section whenever it is possible, because the "Kais" are not so easy to come across, they are usually rewards and they are limited. First of all let's make a more practical example of why you should grind and get a unit's SA to MAX Level and see in what cases it's actually worth doing it in the first place: I'll start with a unit that has an "easy" grind, [Bringer of Light] Fusion Zamasu.
[Wrath of the Absolute God] Fusion Zamasu(the Dokkan-Awakened version of the unit we are farming) has a base ATK stat of 9694; if we add the 2000 boost from the "Free Potential System", he will be at 11.694 ATK. Let's assume we are on a Potara team, under [Super Powered Fusion] SS Goku & SS Vegeta's Leader Skill of 170% ATK; the ATK, before Fusion Zamasu's Passive activates will be at 52.641 and at 115.811 at the start of the turn after he receives the 120% ATK boost from his Passive.
To make the calculations more simple we are going to assume no links are active and check directly what his ATK will be at SA 1 when he launches the Super Attack at 12 Ki and at SA 10:
SA 1 - 115.811 x 1,4 (12 Ki multiplier) = 162.136 (ATK before launching the Super Attack) x 2,8 (SA 1 Super Attack multiplier) = 453.979 ATK
SA 10 - 115.811 x 1,4 = 162.136 x 5,05 (SA 10 multiplier) = 818.783 ATK
His ATK increased by around 80%, this is definitely worth the grind, especially since no resources were really consumed.
A Will Beyond Time Super Saiyan Trunks (Future)(the Dokkan-Awakened version of the unit we are farming) has a base ATK stat of 7985; if we add the 2000 boost from the "Free Potential System", he will be at 9985 ATK. Let's assume we are on a Hybrid Saiyans team, under [Hidden Power Unleashed] Ultimate Gohan's Leader Skill of 130% ATK; the ATK, before SS Trunks' Passive activates, will be at 35.946 and at 44.932 at the start of the turn after he receives the 25% ATK boost from his own Support Passive.
To make the calculations more simple we are going to assume no links are active and check directly what his ATK will be at SA 1 when he launches the Super Attack at 12 Ki and at SA 10:
SA 1 - 44.932 x 1,35 (12 Ki multiplier) = 60.658 (ATK before launching the Super Attack) x 2,3 (SA 1 Super Attack multiplier) = 138.513 ATK
SA 10 - 44.932 x 1,35 = 60.658 x 4,1 (SA 10 multiplier) = 248.698 ATK
His ATK increased by around 79%, but if we look at the raw ATK added it's really not that much even under the current best Leader Skill: this was definitely not worth the grind, especially considering we used 9 Elder Kai Medals to Z-Awaken those units.
Not getting that much of a raw ATK increase because the multipliers and the base ATK are so low is one thing, but another thing is to go through a grind and not get a 100% success rate by feeding the units you farmed into the main one: this is the case, for example, with the new [Outburst of Emotions] SS 2 Vegeta & Bulma. Dokkan Awakening [Mixed Feelings] Vegeta from the Vegeta the Indomitable Warrior Story Event into [Furious Transformation] SS 2 Vegeta & Bulma will result in only a 50% chance because the two units share the same name only when the first one is Dokkan Awakened too. [Outburst of Emotions] SS 2 Vegeta & Bulma is a unit that is worth using "Kais" on both because his grind is unrewarding and because he is a very good unit.
Closing Notes
There is technically another way of raising the Super Attack and that is utilizing the exact same unit as a training partner. This works just fine if we are talking about an in-game unit, but now so much for summonable ones: it's not advised to utilize a "dupe" to raise the Super Attack of a summonable unit unless we already have 5 copies of it, the base unit and four dupes to open all the paths in the Potential System, on which I will expand on a future article.
All the units used as examples in the article were considered to be at 0% in the Potential System (Super Attack Lv. Boost).