/ˈɒpʃənl ruːl/ – Phrase
Definition: quy phạm tùy nghi.
A more thorough explanation: An optional rule is a provision or regulation within a legal framework that is not mandatory or required, but rather discretionary or elective. It provides individuals or entities with the choice to adhere to or implement the rule based on their own judgment or preference.
Example: The requirement to wear a uniform is an optional rule for employees at the company.