This document defines a "parallel" subtype of the multipart Content- Type. This type is syntactically identical to multipart/mixed, but the semantics are different. In particular, in a parallel entity, the order of body parts is not significant.
A common presentation of this type is to display all of the parts simultaneously on hardware and software that are capable of doing so. However, composing agents should be aware that many mail readers will lack this capability and will show the parts serially in any event.