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

Not Administrative: The Cholas 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 — Tamilakam 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 — Tamilakam

Not Administrative · default
Administrative in 1066
Always Administrative

Extra Administrative Realms — Tamilakam is a game rule in Crusader Kings III (Historicity) with 3 settings: Not Administrative (the default), Administrative in 1066 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 — Tamilakam game rule in CK3?: Not Administrative: The Cholas 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 — Tamilakam 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
The Cholas 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_tamilakam_gameplay_not_administrative in game/common and game/events, by the top-level object it sits in.
historicity_extra_admin_tamilakam_gameplay_not_administrative
Administrative in 1066
The Cholas will start with an Administrative Government in 1066 to better reflect their 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_tamilakam_historic_1066 in game/common and game/events, by the top-level object it sits in.
historicity_extra_admin_tamilakam_historic_1066
Always Administrative
The Cholas will always start with an Administrative Government, which is not consistently 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_tamilakam_alt_historic_always in game/common and game/events, by the top-level object it sits in.
historicity_extra_admin_tamilakam_alt_historic_always

Source: game/common/game_rules/00_game_rules.txt line 1467 (key historicity_extra_admin_tamilakam), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.