The transition element defines a single transition class. This element may appear in different places in the document, depending upon the language profile. However in most cases, the transition element will be allowed only in the head of the document. For clarity, a grouping "container" element (such as the layout element in SMIL) may be desired in order to group all of the transition elements together. Since there may be multiple transition classes used in a document, then there may be multiple transition elements in the head of the document.