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.
Doomsday, on the other hand, is an incredibly strong and resilient creature, capable of regenerating from almost any injury and capable of dealing massive damage with his physical abilities. However, he doesn't have the ability to increase his strength or energy, and he doesn't possess as advanced defense skills as Jiren's.
When it comes to a fight between Jiren and Doomsday, Jiren is likely to win due to his ability to increase his strength and energy, as well as his defense skills and ability to teleport. He would also be able to better withstand Doomsday's attacks due to his out-of-the-ordinary durability and stamina. Based on this information, I would say Jiren has about a 90% chance of winning this fight.