Sometimes a lord will call upon those in his fiefs to take up arms and assist him in enforcing the law and common good. Those who answer this call will form up into militia patrols and set out to help keep the roads clear of banditry and will be happy to share stories of their travels should you talk to them. They're not actually bound to a specific area once in the field and it's not uncommon to see a patrol from Foxstead up near Senderfall or a patrol from Gosbur as far away as Windholm or Javiksholm.
Militia Patrols generally have between 15 and 25 men lead by a sheriff and may include any combination of the following:
They will also take prisoners and recruit any they rescue. This can sometimes lead to a militia patrol being only such in name only and have a diverse range of infantry and cavalry with the group numbering into the 50's or greater. When war is declared, the various militia will also take part in the fighting, often approaching the player if he/she's a part of the enemy faction and of a weaker party, battling other patrols, attacking enemy village farmers if they're nearby, and assisting their lords in battle.
It should be noted engaging a militia patrol will cause a deduction in right to rule as well as honor (presumably because you are impeding justice in the land,) and they will never outright attack you unless you're of a considerably weaker party and on atrociously bad terms with their faction. Instead they merely approach you and talk, prompting you to either dismiss them, attack them, or hear about their work.
When a player engages a friendly militia patrol (whether or not the patrol must hail from their fief is another matter entirely,) it is possible to reinforce them with units. This appears to simply bring the patrol back to its original strength if it's been hammered by bad luck. Note that the reinforcement option disappears as soon as the patrol numbers more than 100 units. However, if you already opened the reinforcement menu, you can give as many soldiers as you want to (which is only limited by your own army size). So if you want to create a strong and durable patrol and have a lot of good soldiers you want to get rid of (usually because of high wages), it is recommended to put them all in your army and only then give them to the patrol, all at a time. By doing this you can create patrols numbering more than 200-300 people at the very beginning thus not only increasing their chances to survive and last for long but also decreasing your wages and getting strong allies almost for free.