when setting up in the equipment wizard you'll see the note for
"how much wort is lost when cooling or to the trub?(gunk at bottom of the boiler)"
this is all of your cooling losses, trub, unrecovered wort after the boil, unrecovered wort in the chiller, hoses, etc.
The only fault i have with this is that my trub loss will change from batch to batch based on hop amounts. my IPAs that i brew using pounds of hops have a slightly higher trub loss than my milds/stouts/other odd beers that only use an ounce or two of hops. Or if i'm brewing with fresh or dried whole cones.
Also, If you have a good sealed kettle outlet/dip tube it should be able to recover liquid down to the very bottom. my valve is probably at the 2-3g mark, but with a dip tube that seals well, its able to pull it to nearly empty. my 32g kettles if filled with water, i only have .5qts of unrecoverable volume, even though my valve is much higher than that.