Tier 0 is your average every day Joe Schmoe walking down the street with no ambition, no talent, and no training.

A Tier 1 is your average unprofessional boxer, or maybe a scrappy street fighter.

A Tier 2 is a professional MMA fighter.

A Tier 3 would be a monk from the mountains, a legendary Samurai. So on and so forth.

Tiers 0 - 3, at least regarding training, are what humans are capable of reaching. A great warrior with trustworthy steel and an iron will could fight on equal terms with the other equal tiered opponents who are maxed out in the other 3 regards. Strength, Speed, and Power.