If you mean at what voltage is the cell considered at? It all depends on what your using the cells in! The SF U2 has built in low voltage cutoff which is said to be just over 3.0V. If you were using 2xCR123s, I guess that it would drop to 1.5V per cell. I have a U2 but never used CR123s in it only Li-Ion cells.
I've mentioned this point in several threads lately about my Fluke 189 DMM that uses AA batteries but shows a low battery icon at 1.2105V/cell which means the cells aren't really being depleted fully!