I don't know anything about the Cities battletome, but for all of the other battletomes, battalion warscrolls list required units to take at a higher points cost, in exchange for a special battalion ability and an extra command point at the beginning of the game. You must choose units to take that fit that battalion, but after that, if you have points to spare, you can continue taking anything else that fits your game type. You can even take another battalion, if you have the points.
As far as I understand, you can take them both as long as you have the points.
Battalions are also good for unit placement at the beginning of the game. Entire battalions are deployed at the same time as if they were one "unit". This means that you are more likely to get to choose who gets first turn at the beginning of the game