The protection slightly differs between the first I/O group and the second, but broadly speaking it is protected against voltage spikes.
Here's the protection circuitry for the first I/O group (taken from a generalised description):
The protection circuit has 2 pairs of Schottky diodes, as you can see from the circuit schematic above. (The B-1000-2 is a newer version which is used in Axon and uses a transile instead)
The first protection pair opens at positive voltage over 18.2V and shorts with negative voltage over 0.2V. The maximum current which the circuit can withstand is 70mA, but the protection characteristic is strongly time-dependent – it can withstand significant currents (>1A) for under 1uS, but it will fail if the current is sustained longer.
The second protection pair then further limits the voltage to 10.5V, which is the maximum voltage that the measurement circuit is capable of measuring, but this is only intended to cover the difference between the 10.5V and 18V voltages, not to act as an external current protection if the first pair fails.
The protection is designed primarily against EMC interference; if the device is connected to a source of strong and persistent interference we recommend the use of other external protection, based on varistors and transiles.
For the second I/O group the protection is similar, but the measuring circuitry is much more complex (to provide greater accuracy) which does make it vulnerable to certain specific faults, e.g. if switched to a wrong measuring mode. I will confirm with my coworker, who is unfortunately not available right now, whether there may be any problems with the second group I/Os and provide a description of the protection circuitry later today.
I hope this helps, if you require more details don't hesitate to ask.