This is also a confusing one: The UseDaysMode system preference, that can also be set in the circulation rules is described as:

... when calculating the date due. 

BUT: In set_waiting it's also used to calculate the expiration date for holds:

    my $max_pickup_delay = C4::Context->preference("ReservesMaxPickUpDelay");
    my $cancel_on_holidays = C4::Context->preference('ExpireReservesOnHolidays');

    my $expirationdate = $today->clone;
    $expirationdate->add(days => $max_pickup_delay);

    if ( C4::Context->preference("ExcludeHolidaysFromMaxPickUpDelay") ) {
        my $itemtype = $self->item ? $self->item->effective_itemtype : $self->biblio->itemtype;
        my $daysmode = Koha::CirculationRules->get_effective_daysmode(
            {
                categorycode => $self->borrower->categorycode,
                itemtype     => $itemtype,
                branchcode   => $self->branchcode,
            }
        );
        my $calendar = Koha::Calendar->new( branchcode => $self->branchcode, days_mode => $daysmode );

        $expirationdate = $calendar->days_forward( dt_from_string(), $max_pickup_delay );
    }

I feel like this is quite odd as you can set ExcludeHolidaysFromMaxPickUpDelay to ignore the calendar (and then it will be ignored). 

But if you set it to use the calendar, it can still be ignored when the UseDaysMode setting is set to ignore calendar. So it should probably be:

Ignore the calendar | Use UseDaysMode setting ...

If asked previously, I'd have said don't make this rely on UseDaysMode and just use calendar or not.