What is 12 BE on a rate sheet?

[Dean Hiller]

I got my MLO license this year and then went and also got a company license and then was recently approved by a wholesaler (yes, I am a masochist who likes being overwhelmed with information overload and figuring this all out).  In the rate sheets, I see a column labeled "12 Day BE" or "21 Day BE" or "30 day BE"  with numbers like 

-1.750% -2.625% -3.250% -4.125%

What does this mean exactly?  Is that the cost?