Hey Rick, the -10dbu and +4dbu are for different signal level sensitivities. -10 for inst levels (as they are much lower than line levels, the input circuit is more sensitive (level wise, like mic levels, millivolts)), the +4 is what is considered line level for most devices, designed to take ~1v so the input is less sensitive and can handle higher level signals without distorting/overloading. Many input stages handle both without too much trouble (but depends). It's all about getting the gain structure right so there is the best signal to noise ratio.
So with your ISP, as MJMP uses his, the input is set to -10 to handle (provide more gain) the lower inst level signal, as the MP1 A out he uses into chan2 is line level, he sets that to +4.
In the loop you can adjust the loop control to be inst or line level, it's a dual ganged reversed pot so turning up the send turns down the return (as both send and return are controlled by that pot (which is 2 pots working in opposite directions to one another)). The advantage of having it in the loop is the output stays stereo. MJMPs setup uses just MP1 A out then feeds it on to other stuff so is mono at this point.