Solvent Accessibility, the 20% cut-off method
Its 1.  Below cutoff, buried, above cutoff accessible. 

Paper 1: "A cutoff of 20% was used to define the two states, buried or exposed. With this definition, the dataset was, roughly, evenly split between the two states."  

Only two states are possible: solvent accessible and buried.  

Paper 2: "A given residue is defined as exposed (e) if its RSA is larger than the cutoff
value, and otherwise it is defined as buried (b)."

Abstract for Paper 3: a cutoff of 20% for two-state definition of solvent accessibility.  

If it were def #2 it would be a three state definition.

Paper 4: "On average, 15% of residues in small proteins and 32% in larger ones may be classed as “buried residues”, having less than 5% of their surface accessible to the solvent..."

This paper, by the dean of structural analysis Cyrus Chothia, uses a 5% cutoff, not 20%...

The abstract goes on to say... "The accessibilities of most other residues are evenly distributed in the range 5 to 50%."  

This passage hints that SA doesn't even go up to 80%.  Just considering that you often won't get more than say 60% with this calculation.  I'm just guessing; but the thought being that unless you are at a terminus of the protein, which is often disordered and doesn't show up in a crystal structure, you will have two adjoining amino acids for each residue - just the solvent accesible area taken up by the contact with neighbors could easily be 20% of total.