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.

iMessage must be enabled to configure subject settings

You can enable subjects for composing with iMessage off, but you can only see/change the value in Settings.app when iMessage is on.

iPad - use “shift+tab” to switch fields

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