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

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

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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

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