Disclaimer: The reason I made this guide is because the game doesn't explain things too well.
I couldn't find any legitimate information to help me out with Unit-based skills. And since the customization in this game has no limitations, I figured you can use this guide to help you try out some wacky combinations. However, this guide doesn't have any visual demonstrations. So, if you're really curious, you might have to test them out for yourselves.

The Super Units are collaboration powers used by a variety of characters from the Dragon Ball Multiverse. They come in two types: Super Unit Attack & Super Unit Effect.
The Super Unit Attack is basically a Team Ultimate Move that inflicts a massive amount of damage on the opposing team's Health gauge. The Super Unit Effect doesn't deal any damage, but applies strong buff effects for your team or debuff effects to your opponent's team.

In order to use a Super Unit, you'll have to equip a Super Unit special item to a set number of team members in your Card Deck. And they have to be equipped sequentially: L, S1, S2, etc. Otherwise, they won't be registered in that specific order. Additionally, your Hero Energy gauge has to be at maximum in order to use it, and only a limited amount of Hero Energy can be generated per turn.

Here's a list of some skills that I used to activate the Super Units faster:

• Overflowing Energy
  
• Power Reserve
  
• Natural Gifts
  
• Awe-Inspiring Super Saiyan 3
  
• Villain of the Wasteland
  
• Subjugated
  
• Surefire Victory
  
• Call for Help
  
• Collected Fusion Warrior
  
• Here to Defeat You

Lastly, each character belongs to a certain Unit Position of L and S1-S4. Down below is a list of characters that I've already deciphered. I chose these characters specifically based on the personality traits that were displayed during each Super Unit. The word Unknown simply means that any character can fill that spot instead. I also had to edit a few of the Unit names due to mistranslations, typos, and other errors.

Furthermore, these are only the Super Units that I've unlocked so far. There's bound to be several more still left in the game. So expect this to be updated in the future, albeit not very frequently.
The rest of you can take it from here! Any contribution here can make a difference!

Father-Son Kamehameha - Gohan (L), Goku (S1), Vegeta (S2)

Hope for the Future - Trunks (L), Goku (S1), Avatar (S2)

Ultimate Kamehameha - Goku (L), Trunks (S1), Pan (S2)

Ultimate Awakening - Gogeta (L), Avatar (S1), Gotenks (S2)

Perfect, Parental Bond - Goku (L), Gohan (S1), Goten (S2)

Saiyan Determination - Bardock (L), Tora (S1), Fasha (S2)

Dawn of the Hero - Goku (L), Gohan (S1), Vegeta (S2)

Grim Reaper of Justice - Gotenks (L), Unknown (S1), Unknown (S2)

Ultimate Spirit Sword - Vegito (L), Gohan (S1), Gotenks (S2)

Strongest Super Warrior - Goku (L), Vegeta (S1), Avatar (S2)

Demonic Rampage - Broly (L), Goku (S1), Vegeta (S2)

Godhood Transcended - Goku (L), Vegeta (S1), Beerus (S2)

Unleashed Instincts - Gohan (L), Goku (S1), Vegeta (S2)

Deity-Level Battle - Champa (L), Goku (S1), Beerus (S2)

Proud Saiyan Bloodline - Vegeta (L), Goku (S1), Unknown (S2)

Fusion Warriors of the Time Patrol - Gogeta (L), Gotenks (S1), Supreme Kai of Time (S2)

Universe 7's Ultimate Warriors - Vegeta (L), Goku (S1), Gohan (S2)

Legendary Demon - Broly (L), Unknown (S1), Unknown (S2)

Ginyu Force Pose - Ginyu (L), Recoome (S1), Burter (S2), Jeice (S3), Guldo (S4)

The Super Saiyan of Legend - Bardock (L), Goku (S1), Gohan (S2)

Heroes of the World - Gogeta (L), Avatar (S1), Gotenks (S2)

Xeno Super Warriors - Goku (L), Vegeta (S1), Gohan (S2)

Ultimate Soul Punisher - Gogeta (L), Gotenks (S1), Pikkon (S2)

Galactic Warriors - Bojack (L), Zangya (S1), Bujin (S2), Kogu (S3), Bido (S4)

Turles's Crusher Corps - Turles (L), Almond (S1), Daiz (S2)

Dark Empire Rebellion - Goku (L), Vegeta (S1), Gotenks (S2)