A formation with Extra Armour gains a +1 Armour Value while over half strength. Once it drops to half strength or less then the bonus is lost.
To represent the formation on the tabletop, ensure some of the front rank models are equipped with armour or use some extra shields at the back of the unit to remind yourself and your opponent
Taking Macedonian list as an example:
It only costs 1 point to upgrade Thessalian cavalry from unarmored to light armor.
It only costs 1 point to upgrade Companion cavalry from light to heavy armor.
Yet in both cases it costs 20 points per unit to get this extra armor.
It seems only worth it if you are in a unit larger than 20.
Why would any do that?
Wouldn’t 0.5 points per figure be better?
That way if you have an 8 figure unit of cavalry they pay 4 points, the same rate as normal armor improvements
The idea behind extra armour is to represent the formations which may, or may not, depending on interpretation, have worn armour. Yes its more expensive, but in the case of a large war band, its good value. It also saves fiddling about with combined formations and keeping track of who has armour in a unit and who doesn’t.
The rule has also been used in some army lists to give additional armour which may again, represent better armoured nobles, or the wealthy, taking a front seat in the fighting.
Where we are pretty sure armour was used, troops have access to the kit at the recommended points value
In the two options you mention, the units can take the Extra Armour in addition to the other armour, thus improving their armour save still further which exponentially is much better and as noted we don’t know if they had it or not (or perhaps not sure how to interpret it in game terms) so there is an extra price attached, if you want that extra upgrade (or historically, had the coin to buy the stuff in the first place! 🙂