[Cohen]

Each Province Defence Point is at least 1 men belonging to the Troop selection.

At least 1 men: It depends from the nation and the kind of troop you get.

You've 1 commander til 19 PD points.
You've 2 commanders from 20 PD points.
You can have no more than 125 PD points in a single province.

You've troop type 1 til 19 PD points, the 20th point changes the type of units from now on.
(ie for Abysya, a PD 10 means 10 humanbred and 1 warlord, a PD 20 means 19 humanbred, 1 abysyan infrantry and 2 warlords, a PD of 25 means 19 humanbred, 6 abysyan infantry and 2 warlords).

When you have at least 10 points of PD in a province, this is a sufficient force to patrol the province against enemy hiding/sneaking, thus poor efficiently.

Each PD point costs it's numeric value. (The first costs 1, the second 2, the third 3 ... so on to have a 5 PD value you spend 15, to have a 10 PD value you spend 55).

Usually PD is used to be set to at least 1 point, to prevent an enemy scout to take over the province, and in every case to watch the battle so you know exactly the enemy army and how's deployed and scripted.
It could be worthy to raise it to 5 points about to defend from Call of the Winds/Wilds or such spells.

There're other uses in specific cases however, if you've strong units in PD.