The UL of each entry in the Essence Element Keys Register must begin with
Note that some Leaf ULs in the Essence Element Keys Register contain one or more Byte values
0x7f. These are “wildcards” that are substituted in implementations by a range of permissible values specified in a defining document. For example, SMPTE ST 381 specifies that Byte 14 of the Frame-wrapped MPEG Data Element is set to the number of Data Elements in the Data Item of a Content Package, and Byte 16 is an index to make each instance of this element type unique.