What is the ±7% troop advantage in Game of Thrones: Dragonfire?

When your troop type counters the enemy's, you get a +7% advantage (you deal more, they deal less). The other way round you're at −7%. Otherwise it's neutral (no effect).

The counter wheel, confirmed word for word in game:

Cavalry > Shields > Archers > Spearmen > Cavalry (and round it goes)

• Cavalry beats Shields, loses to Spearmen.
• Shields beat Archers, lose to Cavalry.
• Archers beat Spearmen, lose to Shields.
• Spearmen beat Cavalry, lose to Archers.
• Siege: weak against everything (but deals extra damage to tile durability).

Don't read it as a one-off bonus: the advantage applies to every single hit, for the whole fight, and both sides have their own. Between the one who has it (+7%) and the one who suffers it (−7%), that's a 14-point swing on all damage, continuously.

That said, affinity is worth +20% and auras up to −27.5%: the ±7% is still smaller than the big levers, and above all you only control it when you know what you're facing. So: take it the moment you can see the target, but don't build your strategy on a counter you haven't scouted.