What is the +20% affinity bonus on a dragon in Dragonfire?

Every dragon has an affinity per troop type (Spearmen, Archers, Cavalry, Shields, Siege). Three cases:

• Positive (🟢 green): +20% to the dragon's combat stats for that type. Shown as "+20%" in the formation screen.
• Neutral: no effect (shown as "No bonus").
• Negative (🔴 red): lowers the dragon's stats.

Since the troop type is chosen for the whole army — one selector, all 3 dragons carry it — the ideal is to line up 3 dragons that are all green for the same type.

One note: the Siege affinity has no impact in dragon combat, since you never field Siege troops there. Green or red Siege is cosmetic as far as fighting goes.