Optional
changeUsed to track a comment across iterations. This value can be found by looking at the iteration's changes list. Must be set for pull requests with iteration support. Otherwise, it's not required for 'legacy' pull requests.
Optional
iterationThe iteration context being viewed when the thread was created.
Optional
trackingThe criteria used to track this thread. If this property is filled out when the thread is returned, then the thread has been tracked from its original location using the given criteria.
Comment thread context contains details about what diffs were being viewed at the time of thread creation and whether or not the thread has been tracked from that original diff.