According to the manual (p. 181):
"Each time the switch is toggled (Castle short press) towards the left, it scrolls to the next format on the left MPD (in sequence introduced earlier, 1-2-3).
Each time the switch is toggled (Castle short press) towards the right, it first brings up the TEWS page. Then each subsequent toggle right scrolls to the next format on the right MPD (in sequence introduced earlier, 1-2-3).
When the Castle switch is toggled towards the MPCD, it presents the ADI. Subsequent toggles scrolls through the programmed formats as long as the next press is within 5 seconds."
The behaviour gets inaccurate if selecting only 2 modes instead of 3:
If you only have modes 1 and 2, then the 3rd press will bring up mode 2 again. You need another press of the switch to go back to mode 1.
If you only have modes 1 and 3, then the 2nd press will go to mode 1 again, and only the 3rd press will bring mode 3. Etc for only modes 2 and 3.
Basically, it feels like it's always scrolling through 3 modes (regardless of you having programmed only 2 or even 1). Expected behaviour would be to only scroll through the actual number of modes selected.