Yes the U designation is the card number starting at 0 for the first slot in the rack, the G is the buffer memory for example U0/G12 would mean card zero buffer memory 12 (you need to look at the manual for the allocation of the buffer memories, Usually Buffer memories (BFM for the analogue channels are from 10 to what ever (depending on the number of channels there can be over a hundred BFM' for all sorts of control/information i.e. filters, diagnostics).
Certain BFM's require writing to others you read, there are other functions that do the same thing the TO/FROM functions can be used TO means write to the BFM memory the FROM is fetch data from the BFM, it also allows more than one location to be written or read from for example FROM 0, 10, 4, D100 means read BFM memory from card 0, BFM10 and 4 registers (BFM10,11,12,13) and store them in D100-D103.