Eight of the twelve universes were expected to participate in the Tournament of Power when it began. Only Universes 8, 5, 12, and 1 were not affected. The difference between mandatory and exemption attendance was due to the Mortal Levels of these universes.
There's still some confusion among Dragon Ball fans about what Mortal Levels refer to. Many fans believe it is about the average power level, but this is a myth. The true meaning of Mortal Level and the position of each Universe on its scale is a little more complicated than that.
What Determines a Universe's Mortal Level?
A common misconception about Mortal Levels is that they represent the average power level of a universe. This might have come from the fact that Mortal Levels are measured based on the "quality" of the mortals of a given universe. Since this is Dragon Ball, many fans probably misinterpreted "quality" as "average power level." Strength is part of how Mortal Levels are measured, but it's not the only thing.
The real point of Mortal Levels is to measure the quality of life for mortals in a universe. This means promoting the development of adequate civilizations while eliminating inadequate ones; this is where the roles of the Supreme Kai and God of Destruction become important. The Higher the quality of the mortals and the lives they lead, the higher the Mortal Level of their universe.
Zeno and Future Zeno, the Omni Kings of the multiverse, decide on Mortal Levels. When the current Zeno realized how many universes had insufficient Mortal Levels, he decided to wipe out all universes with Mortal Levels lower than seven. However, Goku persuaded him to change his mind and give the universes a chance in the Tournament of Power.
Universe 7, the Z Fighters' universe, has one of the lowest Mortal Levels at 3.18. In the anime, this made it the second-lowest universe, trailing only Universe 9's 1.86; in the manga, they're also ahead of Universe 6, though the Mortal Level for that Universe was never revealed. Their low Mortal Level comes from a mix of the Supreme Kai's lack of intervention in mortal lives and Beerus' wanton destruction of Planets. This mismanagement of their universe brought the number of well-developed planets down to 28, which isn't many.
How Do the Mortal Levels of the 12 Universes Differ From One Another?
Besides Universes 7, 6, and 9, five more universes were scheduled to be erased by Zeno. The other Universes ranked from lowest to highest are Universes 4, 10, 3, and 11. Despite having Mortal Levels higher than Universes 7's, they all ranked below a 7 on the scale, so they were all considered equally worthy of erasure.
Twin universes are notably similar in Mortal Level. However Universes 6 and 7 rank, they are notably only one rank off from each other. Similarly, Universes 3 and 10, 2 and 11, 5 and 8, and 1 and 12 are all only one rank off from each other in terms of Mortal Level. The only exceptions to this pattern are Universes 4 and 9, but they are still similar in the sense that they are anomalous.
If Goku and the gang ever start exploring other universes, the topic of Mortal Levels might come up once more. What exactly the universes exempt from the Tournament of Power did to achieve such high Mortal Levels would be interesting to find out. Finding a villain to battle in such a universe might be challenging, but the plot might manage it.
In any case, the concept of Mortal Levels shouldn't just begin and end with the qualifications for the Tournament of Power. There's too much story potential surrounding the idea of how Mortal Levels are determined and what Universes with higher or lower Mortal Levels look like. For all of the things that need to come back in later arcs of Dragon Ball Super, this is one of the most important ones.