The Superhero Database Classification number, or SHDB Class, is a number that represents the overall 'power' of a character. All traits of a character are used for calculating the Classification.
What it DOESN'T mean
This doesn't mean that a higher class would always beat a lower class character. But the bigger the difference in Class is, the more obvious it is who'll win in a fight.
How is this calculated
( INT^1.3 + (STR*0.5 )^2 + (SPE*0.5)^2 + DUR^1.6 + (POW + (SPS*SPL))^2 + COM^1.8 ) ^ TIER
Super Power Score and Level
Every Super Power has a score (SPS) that is used to calculate the Class. Each Super Power also has 3 levels (SPL). The level is set when connecting that Super Power to a character. The level determines the final score, of the Super Power, being used in the calculation.
And for The Brothers Death, this is because All-Death.
He's stated to much powerful than his two brothers who have reached High 1-A easily.
Yes I know being much powerful than High 1-A doesn't always reach to tier 0 but All-Death has it.
This is what makes The Brothers Death boundless because All-Death. If you make statistics for each individual especially for Small Death and Great Death I agree they won't reach to boundless but All-Death remains at that.