By default, 3Play outputs the open captions in the same format as the source media upload. However M4V format can be ordered as needed for players such as Quicktime and others.
IMPORTANT NOTE: M4V cannot be ordered for Standout and Bottom Bar formats at this time. The only way to accomplish this is to upload the source media as M4V.
See examples for the different types of caption encoding you can order.
Standard Open Captions
Bottom Bar Captions