"There are a few options that are missing in the codex that appear in the index: why is that? Does that mean I can’t use these models in my army anymore?
While the indexes are designed to cover a long history of miniatures, the codexes are designed to give you rules for the current Warhammer 40,000 range. There are a few options in the indexes for some Characters and vehicles that are no longer represented in the Citadel range – certain Dreadnought weapons that don’t come in the box, or some characters on bikes, for example.
Don’t worry though, you can still use all of these in your games if you have these older models. In these instances, use the datasheet from the index, and the most recent points published for that model and its weapons (currently, also in the index).
They still gain all the army wide-bonuses for things like Chapter Tactics and can use Space Marines Stratagems and the like, so such venerable heroes still fit right in with the rest of your army."
Via the Warhammer Community site
So it seems that for some units you will need the Codex and for some units you will need the Index.
Doesn't seem like an ideal situation! Me thinks they may have made some boo-boos are desperately trying to cover their tracks. Considering the amount of "play testing" they crowed about, maybe they needed to spend more time on the typey typey and less time on the "look at us!!". Pretty poor!