Specialized kind of IStream for objects.
When implementing your own objects, you will sometimes need to provide support for persistence. For example, with a custom feature renderer, persistence enables its settings to be written to an MXD file so that when ArcMap is restarted, the layer having custom rendering will be drawn in the same way as previously drawn. The COM concept of streams provides a way of reading and writing data to the persisted storage.
The ObjectStream is designed to allow you to correctly persist collections of ArcObjects objects in which there may be multiple references to a single object. For example if you persist both a Map and a LegendItem to the same stream, both may have references to a single layer. The ObjectStream ensures that the layer is only written to the persistence stream once. If you persist objects without an ObjectStream, you run the risk that the objects will be incorrectly rehydrated.
See the topics on implementing persistence for more details on implementing persistence on your custom objects with IObjectStream.