"Continuous Updating" to me refers to an asynchronous I/O scan.
"Mass I/O Updating" refers to a synchronous I/O scan.
Okay, so what does that mean?
Synchronous I/O updates means that the I/O is scanned (or maybe just the I), then the user program is run, using that 'Copy' of the physical I/O. In the case of seperate I's and O's, the outputs are only 'physically' updated at the end of the user program scan.
With Asynchronous I/O, the I/O is scanned, and updated either immediately, or at the next timeslot for the update, irregardless of the state of the user-program. With Asynchronous systems, you might see input x.y true at the beginning of the user-program scan, but false somewhere else in the user-program scan.