I’m having trouble understanding the instructions on the DISTO D3 concerning the stake out function. 
I can get the distance “a” set, then “b” – then I understand the tool is helping me set the stake at distance “a”.  I don’t understand how you use “b” – also I don’t understand this which I copied from Leica’s site online:  (this wasn’t in the manual that came with the instrument)

Staking out function
Two different distances (a and b) can be entered into the instrument and then subtracted from specific measured lengths.