The galaxy is a vast place with millions of different worlds: ice worlds, desert worlds, hive worlds, feral worlds and more others than can be easily enumerated. The terrain covering these worlds can vary from broad, empty plains to sky-scraping towers of plexiglass and plas-steel, from verdant jungles to icy peaks or baking hot deserts.
One factor is common to all of them, they all have areas seemingly designed to make it difficult to wage war. Of course areas that provide cover where warriors can fight or which present an obstacle to the enemy are vital in war, and these are destined to become the bloodiest battlefields.
How to represent terrain on the battlefield is discussed further in the Battles Book. For now we're going to discuss terrain only in relation to troops moving through it.