Characters receive a Level once they have been validated
The Power Level Chart
This Chart takes into account the things such as training and Natural abilities, as well as stats to make the Level. Trained soilders will, after all, do better than tourists...
Weapons can be used at...
Power Level 1: Can't use any...
Power Level 2+: Can use Light Melee Weapons
Power Level 3+: Can use Light Firearm weapons and Medium Melee weapons.
Power Level 5+: Can use both Medium Firearm Weapons and Heavy Melee weapons.
Power Level 8+: Can use Heavy Firearm Weapons.
Power Level will only increase under certain conditions, such as completing a Mission or after a certain achievment.
All Creatures, Bosses and Monster will be assigned power levels too, as will all NPCs.