iOS (iPhone/iPad) lets you compose a subject for your messages.

Everyone receives subjects, even if they can’t/don't send them

Phone texting (MMS) and iMessage support receiving and displaying a subject line.

Not enabled by default

Tap on “Show Subject Field”

Optional for sending text

If you leave the subject empty, the text is sent normally

If you type only a subject (and leave the “Message” field empty), the subject is used as the message.

iPad - use “shift+tab” to switch fields

The “tab” key inserts a “tab character”, so “shift+tab” is used for switching fields.