As you can find under the table with cavity centers in pdf version of this article:
The table lists the protein’s PDB ID, the ligand considered and the specified cavity center. 22 ligands are similar to hexoses in shape and/or size. The cavity center is the centroid of the reported PDB atom numbers.
And a little later:
The binding-site center is computed as the
hexose pyranose ring centroid for the positive examples,
and as the ligand or empty pocket centroid for the negative
ones. The hexose pyranose-ring atoms are located up to
2.9
̊
A away from the ring’s centroid. Since some atomic
interactions can be important up to 7
̊
A [22], we consider
the binding-site as all protein atoms present within a 10
̊
A radius sphere around the binding center. All other atoms
are discarded.
So basically @aandreev is right. You can read about centroids here