The extent carries the information about the 'size' of a propagation
after it returns: the directly and indirectly invoked servers, and the orphan
detection information for those.
Serial mode is an optimized way for lock inheritance:
no locks among related transactions are necessary if all related
transactions are executed serially with respect to each other.
Information about the transaction context that can be
shipped along with remote request, to make the other side
participate in the transaction present for the current thread in this VM.