The key-system is taken from Krom & Van Erp's catalogue.
In between the basement at the uppermost stupa terrace, there are four galleries,
each with a main wall and a balustrade:
G = gallery / B = balustrade / main = main wall / dec = decorative
I II III & IV are the main inward-facing walls (i.e. backs to the stupa)
The Balustrade reliefs face these across each gallery (i.e. backs to the countryside);
"a" & "b" indicate two ranks of reliefs on the main wall:
Thus Ia is the upper rank of reliefs on the main wall of Gallery 1;
and IIB are the balustrade reliefs of the second gallery