This functionality should be the subject of a connectathon to determine practical solutions.
One possibility could be to leverage the List functionality described above to maintain a state of what was included in a previous content, however this obviously incurs additional overhead on the part of the server, and for many systems - especially those at scale like a national system - this is likely not practical.
Format of the bulk data extract
This then tells the server that we are all finished with the data, and it can be deleted/cleaned up. The server may also include some time based limits where it may only keep it for a set period of time before it automatically cleans it up.
A close relative to the bulk data extract is the subscriptions content - and how these will work in the context of Bulk Directory exchanges needs further experimentation and connectathon experiences.
These could be setup to monitor the directory for realtime changes to resources of interest, and are defined through the use of search parameters