Both and are optional. If there is a element, it must be the first sub-element of asdf.
The element can contain source sub-elements and an optional reference. All elements within exist for the whole duration of the scene. If they contain transform attributes like pos or rot, those values are static. Additional transform elements can be used in the to offset those values dynamically.
The element can contain seq-par elements, as well as \<clip\> <clip-channel> and transform elements. If the element contains multiple sub-elements, it acts like an implicit \<seq\> <seq-par> element.