A2 used as a way of attracting someone's attention, sometimes in a way that is not very polite(用於引起注意,有時顯得不禮貌)嘿,喂
mainly US used as a greeting(用於打招呼)嘿,喂
More examples
Hey! What do you think you're doing?
Hey! Where are you two going?
Hey, you!
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