Here is a web site that does a good job of answering your question.
www.datacottage.com/nch/testing.htm
If this is more technical that you wanted then go with this. As a signal travels down a cable it is "attenuated" or gets weaker. That is why maximum cable length is specified for cable runs.
Far End Xross Talk is the level at which wire pairs will induce current (signals) into each other.
Attenuation minus FEXT gives an more accurate picture of how much effect the FEXT will really have on distorting signals. This is ELFEXT.
Hope this helps.
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