descriptor() [MPEG-2 Semantics]
A descriptor() designation in a table denotes the location of a descriptor loop that may contain zero or more individual descriptors. A descriptor is a variable-length, generally optional field that may be found at one or more locations in an MPEG-2 private table section.
Functionally, it is used to extend the functionality of a table section without having to change the syntax.
Descriptors are published by MPEG, ATSC, ARIB, DVB and SCTE. Generally, it is permissable to mix descriptors, although many DVB descriptors overlap the "User Private" number space of ARIB, ATSC and SCTE, and vice-versa.
If a receiver doesn't understand a descriptor, it ignores it and jumps to the next descriptor in the loop.
See: Descriptors generally