discretionary
/dɪsˈkɹɛʃəˌnɛɹi/ adjective C1- Available at one's discretion; able to be used as one chooses; left to or regulated by one's own discretion or judgment. · discretionary income· discretionary powers
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The United Kingdom occupies a middle ground, where service charges are sometimes included on the bill, but additional tipping remains entirely discretionary.
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