Brother-Sergeant Chronus

[Enemy5]

I'm a little confused at some wording on a special character. At the end of the Vehicles section in the Codex: Space Marines book, there's a listing for Brother-Sergeant Chronus that states, "Must be assigned to one tank in the army." On the character's profile, it does not mention that he is a compulsory choice. Is Chronus compulsory for a valid army, or just another vehicle wargear option?

[Nabterayl]

What that really means is that if you purchase Sgt. Chronus, you must have at least one tank in your army, to which Sgt. Chronus must be assigned. You don't have to have Sgt. Chronus to field a valid army, but you do need a tank to validly include Sgt. Chronus in your army. Thinking of him as a vehicle wargear option (which you can obviously only choose one of per army) is a good way to think about it.

[Tynskel]

you need to read p. 89 and 143 of C: Space Marines.

under p. 89, he's an upgrade, so he doesn't take up a slot in the FOChart. (another clue is that all of the shaded out regions in the army lists of the 5th codexes have been non-FOC choices.)

p.143 is basically just his wargear, points, ect.