Definition of sigmaa

[Chad C. Sines]

I have never been able to find any written information on the meaning of
these two commands when making maps.  Can anyone shed light on this?

evaluate ($sigmaa_flag=false)  { true or false }   { Sigmaa weighting
flag *}

{===>}
 evaluate ($sigmaa_cv=false)   { true or false }     { Sigmaa
cross-validation flag. *}