Each unit entry tells you how to include it. For instance, I think that the taurus has "is a fast attack choice for a codex: IG army" in it's entry. These units can be included in whatever force it says. However, entries in the Armoured Battle Group list, such as armoured fist squads, can only be used in that army lists. Of course, some units can be used both ways (for example, taurus squadrons).