instruction LET for cutler hamer D300

can anyone give me some information on what a LET instruction for cutler hamer plc. I have a machine that im going to convert to a slc 500 and would like to understand old program better befor converting.thank's

It's a COPY instruction. It copies the source value (S =) to the destination address (D =). Think of it as meaning "Let the destination be equal to the source".