The power dynamic, the setting, the content…so much.
If you are a supervisor/boss/loan officer/or in any way have authority over, power to influence, or power to control – don’t flirt.
If you see someone that you think is cute who is alone in an isolated location – don’t flirt.
If you’ve smile at someone or said hi and they turn their eyes away, or go back to reading their book, or put their earbuds back in – don’t flirt.
Don’t comment on physical attributes. That’s not flirting.
In a social setting, when they’re with a group, when they smile back – go ahead, flirt.