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

Not Administrative: Persia 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 — Persia rule disable achievements?

No: none of its 3 settings carries the flag blocks_achievements in patch 1.20.1.2.

Game rule · Historicity

Extra Administrative Realms — Persia

Not Administrative · default
Administrative in 1178
Always Administrative

Extra Administrative Realms — Persia is a game rule in Crusader Kings III (Historicity) with 3 settings: Not Administrative (the default), Administrative in 1178 and Always Administrative. 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 — Persia game rule in CK3?: Not Administrative: Persia 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 — Persia rule disable achievements?: No: none of its 3 settings carries the flag blocks_achievements in patch 1.20.1.2.

3 settings

Not Administrative
Default
Persia 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_persia_gameplay_not_administrative in game/common and game/events, by the top-level object it sits in.
historicity_extra_admin_persia_gameplay_not_administrative
Administrative in 1178
The Kingdom of Persia will start with an Administrative Government in 1178 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_persia_historic_1178 in game/common and game/events, by the top-level object it sits in.
historicity_extra_admin_persia_historic_1178
Always Administrative
The Empire of Persia & Kingdom of Persia will always start with an Administrative Government, which is not reflective of their historical organisation.
The script checks it once, in 1 place
Scripted effects 1
- apply_historic_administrative_game_rule_effect
Every has_game_rule = historicity_extra_admin_persia_alt_historic_always in game/common and game/events, by the top-level object it sits in.
historicity_extra_admin_persia_alt_historic_always

Source: game/common/game_rules/00_game_rules.txt line 1441 (key historicity_extra_admin_persia), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.