Irregulars aren't professional soldiers but, in one way or another, they are associated with the military. They all have enough basic training to fight in an organized way, and may be deployed to defend their home settlement. They don't go to war.

They are built using the Paramilitary Template.

An Auxiliary is an expert who has joined the military to benefit from a permanent job. They are normally L3 NPCs who have has some basic military training, and skills in either a craft of a profession that is relevant to the military - sailor, cook, farrier, blacksmith, stable master, clerk or anything else that might benefit their home base.

In normal circumstances they carry out their main job and stand guard, as required. They are normally equipped with leather armour, buckler, Light Crossbow and a dagger. They are trained to provide missile support if their base is under attack,

Example Auxiliary Profile

Militiamen are the locals that the army can call on to help defend their town or village or to join in searches, hunts and other civic duties of that type. The militia includes all sorts non-military NPCs who have attended training sessions at their local Watchtower / Barracks / Garrison etc. They tend to have a real mixture of equipment, but almost are all trained with spears and darts.

Why do it? Think of the various volunteer 'emergency services'. In the UK we have: The 'Special' Constabulary, RNLI, Coast Watch, Mountain Rescue Teams, and others as well. In this environment, it probably has a minor tax relief and gets him a bit of extra work because he is an 'upstanding member of the community'. Or it might just be because of the free bread and soup given out on training night. There are many potential reasons, and they will vary from one Militia member to another.

Example Militia Profile (commoner)

Example Militia Profile (rogue)