I'm looking at a monograph that at times refers to a tablet with
cuneiform. It shows some references like B13 for the sign AN, "the sky",
  B151 for LUGAL, "king", and so on. In another I find, BA subscript 6
(B318). What are the Bs and the subscripts? Another is SUS with (convex)
marks over the Ss and a numeric subscript of 2. Following the text, a
line is "The zone connecting the upper object contains 4 signs believed
to be BAR+SUS2(B74+B545) "thinning overcast ... SUS2 is the subscripted
2 with marks over the Ss. They are talking about interpreting cuneiforms
on a clay tablet, K8538 in the British Museum.

In a Wikipedia entry for cuneiforms I find items like GE23 (subscripts
are 23) in some descriptions. Same general question. What is GE and what
are the subscripts?

For specific details of the monograph see
<http://www.amazon.com/gp/product/1904623646/ref=cm_rdp_product>. Use
Look Inside and go to page 19, for example. One way to get to page 19 is
search for BABBAR.