They refer to certain resources that are not available outside the process. An example is the input stream of an open file. Consequently, sited objects may not be pickled. Any attempt to pickle values containing sited objects will result in a runtime exception. Moreover, functions that create sited objects are also sited.
Special care has to be taken to avoid runtime errors from attempts of pickling such objects.
This is particularly true when pickling modules. Futures are never pickled. Instead, the pickling operation will block on all unavailable futures that are contained in the value to be pickled. In the case of by-need futures this of course will force evaluation. A failed future will cause an exception, respectively.