I use this way:
Conscript soldier who has been drafted for a specific war -> commoner
Commoner who took up arms (willingly or not) and stayed with that profession for longer and thus also received training/experience -> warrior
Person who has been trained for fighting since his youth instead of a other craft -> fighter
That also means that a 1st level fighter is not some battle hardened veteran. Instead he just finished his formal training which already puts him above most of his untrained peers, but is still less skilled than someone who, for example, did not receive much of formal training but is a veteran of many battles (high level warrior).
Conscript soldier who has been drafted for a specific war -> commoner
Commoner who took up arms (willingly or not) and stayed with that profession for longer and thus also received training/experience -> warrior
Person who has been trained for fighting since his youth instead of a other craft -> fighter
That also means that a 1st level fighter is not some battle hardened veteran. Instead he just finished his formal training which already puts him above most of his untrained peers, but is still less skilled than someone who, for example, did not receive much of formal training but is a veteran of many battles (high level warrior).