optional rule
A rule that can be chosen or applied at one's discretion, often in games, sports, or procedural contexts.
このゲームでは、選択ルールとしてタイムリミットを設けることができる。
In this game, you can set a time limit as an optional rule.
Compound of 選択 (selection, choice) and ルール (rule), a straightforward modern term.