The dataslate specifies exactly what the formation is made up of. It also lists any special rules that applies to models in that formation. You can see it as a mini-supplement, like Iyanden is for codex Eldar. But without any background, images, art, etc. Basically a cheap way for GW to make money by slapping some special rules on a part of an army list, then taking a snapshot of it, print is as PDF and charge people for it.