Here and here and numerous other places I've looked online state that Roman Legions divided their infantry into cohorts based on skill, and that all legions had their cohorts identically manned. The cohorts are described as follows:
Cohort I: Was made up of the elite troops.
Cohort II: Consisted of some of the weaker or newest troops.
Cohort III: No special designation for this unit.
Cohort IV: Another of the four weak cohorts.
Cohort V: Again, no special designation.
Cohort VI: Made up of "The Finest of the Young Men".
Cohort VII: One of the four weak cohorts and a likely place to find trainees and raw recruits.
Cohort VIII: Contained "The Selected Troops".
Cohort IX: One of the four weak cohorts and a likely place to find trainees and raw recruits.
Cohort X: Made up of "The Good Troops".
The problem is, exactly 0 of the sources I've found with such a list cite where it came from. What's more, the dozen or so sites all have it copied the exact same way, indicating they may just be doing circular reporting of one person making it up and putting it on the internet. Is there a primary source/s this is taken from? If so, is there reason to believe it was a long-lasting staple of legion organization? (ie: was it likely to have existed before and/or after the document in question? Was it late republic? Imperial? Late Imperial?) Was it a universal system, or just how a single legion was organized? (ie: can it be said that EVERY VI Cohort of the period was made up of "The Finest Young Men"? Or just that one of the cohorts was always assigned the best of the younger troops?)
I know the 1st cohort is the elite of a legion. It also stands to reason that you'd want the 10th cohort (ostensibly your other flank-cohort) to be made up of better-than-average troops, and more generally that you'd have "training" cohorts where recruits would be sharpened "in the real world" compared to whatever they learned at any basic training they received. Or even that the cohorts were thus organized as some sort of cultural remnant of the old manipular legions. But "makes sense to me" and "something that actual has historical basis" are not the same thing!