Basically PDFs(unless they have changed the fluff, which they may have because GW is GW after all) are the standing army that each planet is expected to maintain. Whenever the Imperium calls for new IG regiments from a planet those regiments are usually selected from the planet's PDF forces. Unless more regiments are requested then there are PDF troops, then the Imperial Govenor just picks more people from the population however he/she sees fit.<br/><br/>I didn't intend for my character to have been in the PDF though, still working on his backstory, just wanted to help add to the setting XD<br/>