When talking about the sub and LFE (Low Frequency Effects) this is a frequency set on the receiver or sub. Frequencies below this setting will be routed to the sub. Some speakers are not capable of going waaaaaay down low, so you set a crossover that will reroute those signals to the sub. Now there are a lot of variations and recommendations on how to do this, and options can vary from receiver to receiver.
Hopefully that is what your were talking about.....I'm sure others will chime in here and confuse the heck out of ya
