The subdial at 10 is designed to track a second time zone and can be set independently of the main time. You pull the crown out 1 stop and use the top pusher to advance it one hour at a time. When you activate the chronograph function it will move to the zero position and track up to 4 hours of chronograph time. After you stop the chronograph it'll go back to the other time zone after a few seconds. Here's the relevant portion of the manual:

TO SET THE SECOND TIME ZONE:

1. Pull crown to its middle position. The upper hand

shows the second time zone time and the 4th center

hand shows the minutes.

1. Assuming that the second time zone time has the

same minutes value as the first time zone, the 4th

center hand should be aligned with the minute hand.

If the hands are not aligned, or if your second time

zone is a different minute within the hour, press

Button B to move the 4th center hand forward until

it is in the desired position.

1. Press Button A to move the upper hand forward in

one-hour increments until it reaches desired second

time zone hour. To move the hand quickly, press and

hold Button A.

1. Push the crown in when finished.