Planetary Defence Forces (or PDF for short) is a term referring to the primary military defence forces of an Imperial world.
The PDF is under the control of the local government rather than direct Imperial control; the raising, training, arming, leadership, etc, is entirely in the hands of the Imperial Governor. All Imperial planets, in general, are required to maintain a standing army in order to maintain government (and thus Imperial) control over the planet against any possible invasion or insurrection. In response to any threat, it is the responsibility of the PDF to engage the enemy until Imperial reinforcements can be mustered from the nearby sector. Thus although rarely respected due to their inherent lack of experience, PDF are widely considered crucial to the Departmento Munitorum as the first line of defence for Imperial worlds against Xenos invasion and Chaos-inspired rebellion. Just as significant is the PDFs' role as the main source of recruits for the Imperial Guard. Whenever required, the best soldiers in a PDF are transferred into the Imperial Guard, receiving further training as they are shipped to their destined location elsewhere in the galaxy. Typically one-tenth of the PDF's personnel are given to the Guard, and these are usually the best soldiers as the penalty for a Governor for giving sub-standard soldiers is death. Those who are even more valuable are often formed into Kill Squads, trained and equipped like Storm Troopers Many PDF forces base their appearance and equipment off of the venerated Cadian regiments. Others base themselves off the appearance of other notable regiments, such as the Tallarn, Mordians and Valhallans, often motivated by similar climates or even due to assistance from those regiments during the founding of the PDF. Still, given the unique and underlying culture of many worlds, PDFs and by extension their Imperial Guard regiments are armed and appear as their world can best equip them, a sign of the diverse cultures that constitute the Imperium as a whole.
Although the PDF defaults all command to any Imperial forces that arrive, there are many possible reasons the PDF or elements of it may join in rebellion against Imperial authority; Chaos or alien (particularly Genestealer) infiltration of the planet, the ambitions of corrupt Planetary Governors and other organisations in charge of the planet, can mean the PDF turns against the off-world Imperial forces. This is a cause of mistrust for PDF in general, and further reason for commanders to prefer to rely on non-local defences. Inquisitors are particularly known for a lack of trust for the local PDF, often quick to have nearby Imperial forces available should need call for it.
SYSTEM DEFENCE FORCES
Many Imperial planets also maintain small independent space fleets, known as System Defence Forces (or SDF), performing similar duties to the Imperial Navy, albeit on a smaller and less well-equipped scale