1. draw attention to something
flag an issue/email/concern
When you ‘flag something’ it means to bring something to the attention of others or to emphasize its importance. You can ‘flag an issue’ or ‘flag something’ of concern. Similar phrases are ‘raise an issue’ or ‘bring to your attention’. This can sometimes be used as a polite way of reminding someone of something: “I’d like to flag that your account is overdue.” This is a phrasal verb more likely used in professional contexts.