The "Mult" box contains the the value by which each pixel will be multiplied after being read in. The default is 1. "Mult" can be negative, for example if this is a background file.

Order of operations: First, the contents of the Add box are added to each pixel in a given file. Then, each pixel is multiplied by the Mult box contents. Finally, the pixels in all the files are added together. Thus, if you had a data file A.raw which was counted for half as long as a background file B.raw, you might set the Add values for both files to be zero, the Mult value for A.raw to be 2, and the Mult value for B.raw to -1.