this data is about the 'form or forms of communication' that are used to express the content of the resource; a closed list of terms is used to identify whether the content of the resource is communicated using text, or spoken words, or still images, etc.
data for this element must be Recorded from a vocabulary (Closed): the term(s) must be appropriate for the resource, even if they are not from the official list of terms
but be sure to check the resource carefully, if the data for the element is Core, or if the data is important for differentiating the resource from a similar resource