From Lexicanum

A μ-class or feudal world is a classification of world existing in a technologically medieval state. The most advanced such worlds possess black powder weaponry. The tithes from these planets are slightly more than those of feral worlds (generally from Solutio Prima to Solutio Extremis), thanks to the development of agriculture. A feudal world typically has a population of 10,000,000 to 500,000,000 people.

Feudal worlds are self-sufficient, and due to dependence on agriculture, a large part of the populations of these worlds are peasants, usually ruled by smaller warrior aristocracies. Feudal worlds are of relatively little use to the Imperium; the true position of their place in the universe may constitute something of a culture shock to the inhabitants, making them poor material for Imperial service - although the warrior nobilities are often a source of recruits for the Space Marines.

Notable Feudal Worlds

• Attila
• Boras Minor
• Chbal (Home of the Bouncer alien creature)
• Molov
• Solstice
• Yamnan

Source

• Warhammer 40,000: Rogue Trader
• Warhammer 40,000 3rd Edition Rulebook

Worlds of the Galaxy
Imperium	Adeptus Astartes Homeworld • Agri World • Cardinal World • Civilised World • Dead World • Death World • Desert World • Feral World • Feudal World • Forge World • Garden World • Hive World • Ice World • Industrial World • Jungle World • Mining World • Ocean World • Paradise World • Penal World • Research Station • Shrine World
Xeno	Xenos World • Daemon World • Exodite World • Tomb World • Maiden World • Crone World • Ork World • Tau Empire • Craftworld