What do A pos and C pos mean?
The # column is the rank of each listed entry for X-ray crystallography — rank 1 means this entry is where X-ray crystallography has the most articles or citations. A pos and C pos show the reciprocal: the position of X-ray crystallography within that entry's own ranked list, by articles and citations respectively. A low value means X-ray crystallography is among the most prominent contributors for that entry.