PRL_STATES_CALLBACK is a function prototype for implementing callback functions for use in PHT_VIRTUAL_DISK calls.
PRL_STATE_PROGRESS_MIN -- The lower bound of the operation completion range.
PRL_STATE_PROGRESS_MAX -- The upper bound of the operation completion range.
PRL_STATE_PROGRESS_COMPLETED -- Indicates that the operation completed successfully.
A value between PRL_STATE_PROGRESS_MIN and PRL_STATE_PROGRESS_MAX indicates that the operation is still in progress. A value greater than PRL_STATE_PROGRESS_COMPLETED indicates that the operation completed successfully. A negative value indicates an error.
A pointer to a callback function can be passed to another function that accepts it as a parameter. The operation initiated by that function will be performed in the background thread, which will call the callback function (possibly more than once) passing to it intermediate results or the final result of the operation.