There is some confusion in the question.
Solvent accessibility of a residue, i.e. the area accessible to the solvent (water), depends on the 3D structure of the protein. It's not a property of an amino-acid (chemical compound such as alanine, leucine, ...).
There may be a correlation -- amino-acids with hydrophobic side chains are more likely to be buried, but in general amino-acids (chemical compounds) cannot be classified as buried or exposed.
I don't have access to the first article, but I tried running the prediction server described in the second linked article, and I got a result
Porter (Secondary Structure):
H = Helix (DSSP classes H, G and I)
E = Strand (DSSP classes E and B)
C = Coil (DSSP classes S, T and .)
PaleAle (Relative Solvent Accessibility):
B = very buried (<=4% accessible)
b = somewhat buried (>4% and <=25% accessible)
e = somewhat exposed (>25% and <=50% accessible)
E = very exposed (>50% accessible)
In one place in the sequence you can see
QQ, predicted as
So clearly glutamine is NOT assigned to a particular RSA class.
Dividing RSA (relative solvent accessibility, 0-100%) into categories is arbitrary, I don't think there is any widely used convention. The easiest way would be to just use numeric values.