What is the default Extra Administrative Realms — Arabian Empire game rule in CK3?

Not Administrative: The Arabian Empire will not have an Administrative Government in any start date, in line with expected gameplay. (patch 1.20.1.2)

Does changing the Extra Administrative Realms — Arabian Empire rule disable achievements?

No: none of its 2 settings carries the flag blocks_achievements in patch 1.20.1.2.

Game rule · Historicity

Extra Administrative Realms — Arabian Empire

Not Administrative · default
Administrative in 867

Extra Administrative Realms — Arabian Empire is a game rule in Crusader Kings III (Historicity) with 2 settings: Not Administrative (the default) and Administrative in 867. 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 Extra Administrative Realms — Arabian Empire game rule in CK3?: Not Administrative: The Arabian Empire will not have an Administrative Government in any start date, in line with expected gameplay. (patch 1.20.1.2)
Does changing the Extra Administrative Realms — Arabian Empire rule disable achievements?: No: none of its 2 settings carries the flag blocks_achievements in patch 1.20.1.2.

2 settings

Not Administrative
Default
The Arabian Empire will not have an Administrative Government in any start date, in line with expected gameplay.
The script checks it once, in 1 place
Scripted effects 1
- apply_historic_administrative_game_rule_effect
Every has_game_rule = historicity_extra_admin_arabia_gameplay_not_administrative in game/common and game/events, by the top-level object it sits in.
historicity_extra_admin_arabia_gameplay_not_administrative
Administrative in 867
The Arabian Empire will start with an Administrative Government in 867 to better reflect its historical organisation, though it may not function as well in play.
The script checks it once, in 1 place
Scripted effects 1
- apply_historic_administrative_game_rule_effect
Every has_game_rule = historicity_extra_admin_arabia_historic_867 in game/common and game/events, by the top-level object it sits in.
historicity_extra_admin_arabia_historic_867

Source: game/common/game_rules/00_game_rules.txt line 1408 (key historicity_extra_admin_arabia), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.