A newbie asks...

[Kobal2]

Yes, PD appears immediately as you buy it. More buying, more units, effective immediately. You cannot see it outside of battle (the manual tells you what nations gets what units though), nor can you take units from it or move it around or get your money back in any way. MA Tien Chi also gains points of PD overtime as a side effect of its dominion, but it's a really marginal increase.

The PD soldiers replenish automatically to full at the end of the battle if you win it.

Not sure what you're talking about re: "a few odd units that I don't recall building", but it's not PD. Some commanders automatically generate troops around them though - for example, the Celestial General creates human soldiers wherever he goes, the Vampire Queen summons vampires once in a blue moon etc... It's usually mentioned in the unit's flavour text.

[Gandalf Parker]

Province Defence is militia. They are the farmers and bakers and blacksmiths running out to defend their province. They are untrained, no formations, rabble that charges the enemy. They dont show up because they arent an "army" until attacked. And even then they arent much of one.

But they can hold off many events. They can spot scouts and spies and seducers and assassins moving thru your area. They dont need monthly upkeep charges. And they dont get used up as long as you own the province no matter how many times they fight.

Always get at least one point in PD. That gives you a commander that can at least tell you who attacked if someone takes the province from you. Otherwise you get no report.

[Bananadine]

One time I took over half a guy's nation with scouts. He was mad

[thejeff]

You see them only in battle. They're there as soon as you pay for them and will return in every battle. You'll never see them on the overland map, you can't put them in regular armies and take them away.

The few odd units may have fled from other battles?

[JonBrave]

As ever, thanks, much clearer! I suspect the random units are down to any number of possible mental oversights But clearly not PD.

So if I win a battle in my PD province, my PD is straight back to whatever I bought. If I lose --- or the AI loses against me --- do I assume the "province winning" power inherits PD back at 0?

[Gandalf Parker]

Yes, if someone wins a province it wipes out PD and Temples. Those are considered to be loyal to your pretender.
If you are going to lose a province and want to "salt the earth" (deny them resources) then castles, labs, population, and peace (unrest) have to be destroyed manually by you before you leave.

[JonBrave]

Gandalf, do you ever sleep? Are you ever not here?

[Kobal2]

Yeah, the equation is fairly simple : if after the battle, you still own the province, PD is immediately back to full. This happens even if that province takes multiple successive attacks BTW - PD will replenish in between each fight.

If at the end of the battle, someone else owns the province, PD is set back to 0 for everyone - the loser won't get back his PD by retaking the province, either. So if for instance you teleport a guy in, wipe the PD, then your thug gets bumped off by an invading army moving in the province in the same turn, your opponent will have kept his province but lost all the PD.
Which is why few people invest too much into it generally, since a decent raiding party can cost you all that sunk investment.

General rules*: 1 sees off non-giant scout takeovers, 5 takes care of single casts of call of the wind / arouse hunger / call of the wild, 10 will catch a scout sneaking around once a decade and 15 will usually resist random barbarian attacks and the like which can be handy if you've got high misfortune scales.

Anything above that is probably a waste for general purposes, although dropping some quick PD just before a major defensive battle can be sensible if you have spare change and the additional soldiers won't be a liability.

*Disclaimer: these numbers are general values provided for information purposes only and the exact thresholds vary in practice depending on the soldiers your PD gives you. Your mileage may vary. By purchasing this PD you implicitly accept that it will suck, as well as any and all consequences of such suckage. Do not feed your PD after midnight. Warranty void if removed. If your PD lasts for more than 4 hours, consult a physician.

[thejeff]

Dropping the quick PD in before a battle can be especially good if you can teleport a few mages in, but can't get any troops for them to hide behind.

They may be able to buff the PD to a useful level or they may just be able to rain destruction from behind disposable meat-shields.

[sector24]

PD is available instantly, but never moves and only defends the province it is built in. It's not a "unit" and has no upkeep, it's just static defense that joins the battle if you are attacked.