Weapon Proficiency

Weapon Proficiency determines how well a creature can handle certain types of weapon, which may add penalties or bonuses, depending on the skill level. Most non-warrior group classes may become proficient (1 slot) in their weapon array, paladins & rangers can specialize (2 slots), only fighters are able to achieve grand master (5 slots). Different classes also have various fighting style investments.

Slots gained with level by class group
Growth rate differs based on class group, multi-classes use only the class of the fastest progression; dual-classes only gain slots from the current active class.

Allowed slots investments by class kit
Max slots per weapon differs based on class kit, fighter, ranger multi-classes may specialize (2 slots), other combinations may only be proficient (1 slot); dual-classes gives the better of the two caps, with fighter dualing are allowed up to 5 slots, but beware of risking wasting and permanently losing any duplicate slot expenditure, as the slots spent are not combined, only chosen the better value if you take the same proficiency in both classes, so keep track of what you have spent. Fighting styles have different investment values.