Action Message Template
The 'Message Subject' is the subject line on the message to the target (usually email). If a message is edable, the subject will also be editable.
Thank You Page Template
The 'Subject' of the Thank You Page is used as the heading on the Thank You Page.
Thank You Email Template
The 'Subject' of the Thank You Email is the subject line of the email to participants after thay have completed the Action Edition.