The manual... http://www.retrevo.com/support/Denon-AVR-2310CI-Receivers-manual/id/23452bh638/t/2/
I'm pretty sure you mean "LFE + Main".
You need to identify the speakers to the receiver as Large or Small regarding their bass-handling capacity more than their actual size.
"LFE" removes the possibly overloading bass in a given speaker channel containing 'small' speakers and sends it to the subwoofer.
"LFE + Main" is sort of explained on page 29 "Bass Setting". The intent of LFE+Main is to task your subwoofer with the bass that ALSO exists on all other speaker channels regardless of their ability to handle it all by themselves. As you can imagine, this might make the subwoofer respond to sonic events that are actually supposed to be directional from, say, the rear channel(s). In that case the event might seem to come from wherever the sub is located instead of the intended direction; or at the very least it might seem to come from both places which is probably not what the sound engineers intended when they recorded the piece.
Having full-range speakers 360-degrees around PLUS a subwoofer, I don't subscribe to the 'bass-is-non-directional' school of thought. If you've ever felt the concussion from and subconsciously glanced toward one of the speakers after, say, a huge explosion, the system is working.