To help you enter email faster, there are various defaults.
Choose a default subject, and future emails will have this default subject.
Same for Brief Body, Thread ID.
If you have several emails with the same thread ID, you can update the subject so that all the subjects are the same. Press Update Subject.
When you enter a email with the same subject as a previous email, the Thread_ID is the same as the previous email. Otherwise, it will be the default value set above. If no default is set, it will be the same as the Email_ID. You should be careful in setting a default Thread ID.