What is the default Impact of Advantage game rule in CK3?

5: Each point of Advantage you have over your opponent in Battles will increase Damage by 5%. (patch 1.20.1.2)

Does changing the Impact of Advantage rule disable achievements?

No: none of its 5 settings carries the flag blocks_achievements in patch 1.20.1.2.

Game rule · Tweaks

Impact of Advantage

1
2
5 · default
7
10

Impact of Advantage is a game rule in Crusader Kings III (Tweaks) with 5 settings: 1, 2, 5 (the default), 7 and 10. None of its settings disables achievements. Patch 1.20.1.2, from game/common/game_rules/00_game_rules.txt.

This section as JSON: /raw/game-rules.json

Quick answers (2)

What is the default Impact of Advantage game rule in CK3?: 5: Each point of Advantage you have over your opponent in Battles will increase Damage by 5%. (patch 1.20.1.2)
Does changing the Impact of Advantage rule disable achievements?: No: none of its 5 settings carries the flag blocks_achievements in patch 1.20.1.2.

5 settings

1
Advantage damage +1% per advantage
Each point of Advantage you have over your opponent in Battles will increase Damage by 1%
advantage_damage_effect_1
2
Advantage damage +2% per advantage
Each point of Advantage you have over your opponent in Battles will increase Damage by 2%
advantage_damage_effect_2
5
DefaultAdvantage damage +5% per advantage
Each point of Advantage you have over your opponent in Battles will increase Damage by 5%
advantage_damage_effect_5
7
Advantage damage +7% per advantage
Each point of Advantage you have over your opponent in Battles will increase Damage by 7%
advantage_damage_effect_7
10
Advantage damage +10% per advantage
Each point of Advantage you have over your opponent in Battles will increase Damage by 10%
advantage_damage_effect_10

Source: game/common/game_rules/00_game_rules.txt line 1570 (key advantage_damage_effect), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.