There are 91 kosher and 85 non-kosher structures and situations of a sukkah mentioned in Shas.  

The numerical value of the word “סוכה” when spelled in full, is 91, indicating a proper sukkah.

The numerical value of the word “סכה” when spelled without “ו,” is 85, indicating a sukkah which lacks kashrus.

~ The Vilna Gaon