From ErfWiki

Proposed Canon

A combat bonus (also referred to as force multiplier)^Erf-b1-p125 is a positive circumstantial influence that increases unit stats and makes it more efficient in combat.

Bonuses can derive from^Erf-b1-p125:

• Leadership
• Terrain
• Artifacts
• Special abilities
• Spell buffs
• Dance-fighting

Bonuses are very important in the combat system of Erfworld. In order to win a side should increase bonuses for their units while decreasing bonuses to enemy units.

Speculation

Bonuses are probably multiplicative^Erf-b1-p125 - bonuses of various types and even of the same type add up^Erf-b1-p113. There is also a stack bonus for stacked units that maxes out with 8 units.