What are the MPEG "object descriptors"?

The ESM part of Systems also specifies means to identify and name elementary streams so that they can be referred to in a scene description and be attached to individual objects. This association is performed in object descriptors that are transmitted in their own elementary streams. Object descriptors are separate from the scene description itself, thus simplifying editing and remultiplexing of MPEG-4 content. The descriptors associate audio-visual objects, more precisely, nodes in the scene to elementary stream identifiers. An additional mapping is required to resolve these identifiers to actual transport layer "channels" (e.g., port numbers). How this mapping is performed depends on the delivery layer instance that is actually used. In accordance with the goal of allowing the use of any delivery layer, MPEG does not define this mapping but rather expects parties that define these delivery layers to define how MPEG-4 content should be mapped to their design in a way that they consider most appropriate.