A single SMS message is generally limited to 160 characters, which gives it the potential to be a very concise means of communication. If, however, you feel that the point of your message will be hard to convey in such small snippets, Hangouts has a feature for you.
Hangouts allows users to specify a subject for SMS messages. Simply hold the send button for a moment, and a subject box will appear as shown below.
Tip: Hold 'Send' In A Hangouts SMS Conversation To Specify A Subject was written by the awesome team at Android Police.