Enumeration resume handles are identifiers for the actual resume key contained in the instance data for the function. This is required for security, interoperabilty, and to simplify the caller code for the function.
If a NULL is passed for the pointer to the resume handle, no handle is stored and the enumeration search cannot be continued. This is useful in cases where the application does not want to enumerate all the items.
If an error is returned from an enumeration call, the resume handle must be treated as invalid and not used for any subsequent enumeration calls. For example, the enumeration should be restarted from the beginning.