In the context of electronic mail, the term designates a marker applied to a message to denote its importance or to remind the user to take specific action concerning that email at a later time. For example, a recipient might apply this indicator to an email requesting a response to ensure a reply is sent by a given deadline.
This feature enhances email management by visually highlighting items requiring immediate attention or future follow-up. The implementation of this functionality varies across different email clients, but generally includes options to set different priority levels or assign due dates. Historically, it is analogous to placing a physical sticker on a paper document to draw attention to it.