The way I see fighters is thus:

At level one they are the new recruits, they will have far more potential than warriors or other NPC classes, but they are the new blood.

At level 3-4 we are looking at professional soldiers, they fight and drill for a living, they will be brave, smart and disciplined; able to adapt to situations as they come.

Levels 5-6 are elite soldiers, the finest troops in the regular army or specialists from a crack company. They are the private guards of Nobility or powerful organisations.

7-8 are officers and men with years of combat experience, they've nearly seen it all and very little would surprise them. You might find small units of veterans like these, they will be renound units that have never lost a fight and are the pride of a nation. These are seasoned adventurers that have seen more than their fair share of carnage.

9-10 are the most capable men and women you are likely to find, they can dispatch most threats with out breaking out in a sweat. They will be the heroes that companies rally around, the champions of villages or cities. Songs will be sung about them in local taverns and places they fight in.

11-15 These are the champions of armies, elected to do single combat to win honour or glory, these are the men and women that are sung about across a country. These are king's personal go-to men. Adventures of such renound that kings and queens will know their names personally and follow them with interest.

16-20 Legends in their own right, these are the sorts of people that songs are sung about should they leave any witnesses to their deeds. A kingdom might have a few fighters this level and if they are lucky they will be elect to use their powers for good. A force of nature that could only be stopped by some immense combined effort. They will be the rulers or champions of the most powerful noble houses or Royal lines, or mercenaries that decide the fate of battles and wars.

21+ Far beyond mortal limits, they can barely be touched by any other man. They are killing machines that fear nothing and are known to the Gods, they direct the fate of worlds and planes