A Subject Line is the introduction that identifies the emails intent. This subject line, displayed to the email user or recipient when they look at their list of messages in their inbox, should tell the recipient what the message is about, what the sender wants to convey.

What is a subject line in a text?

It literally means show the subject field in Messages. When on, youll see a subject line above iMessage when you go to send a text. When you send a text with a subject, it appears bolded and acts as a kind of title for the body of the text message.