I have the DLS45, which I like very much. All DLS units work the same way. I called Iota tech support and asked them this very question . . . and forgot the answer.
You can either g00gle for the phone number (or maybe one of the Search Boyz will provide a link) and call Iota tech support and ask them, or listen to me scratch my head and make the following guess:
The jumper in question will ENABLE 14.2 boost mode, which will then obey EITHER the internal algorythm regarding batt voltage levels OR a timed maximum which I THINK is something like either 8 hours or maybe thirty minutes . . . who can remember?
This algorythm is a multi-stage charge (that means more than one stage), while the timer reflects that while the DLS can be used as a power supply at 13.6 with no limit, it cannot at 14.2.
End of guess.