With the -z flag zero data values are not crossed. This means that if a zero category value occurs in any input data layer, the combination is assigned to category zero in the resulting map layer, even if other data layers contain non-zero data. In the example given above, use of the -z option would cause 3 categories to be generated instead of 5.
If the -z flag is not specified, then map layer combinations in which not all category values are zero will be assigned a unique category value in the resulting map layer.
Category values in the new output map layer will be the cross-product of the category values from these existing input map layers.
          map1   map2
          ___________
           0      1
           0      2
           1      1
           1      2
           2      4
          map1   map2   output
          ____________________
           0      1       1
           0      2       2
           1      1       3
           1      2       4
           2      4       5
          category   category
          value      label
          ______________________________
             1       layer1(0) layer2(1)
             2       layer1(0) layer2(2)
             3       layer1(1) layer2(1)
             4       layer1(1) layer2(2)
             5       layer1(2) layer2(4)
Last changed: $Date: 2016-01-28 12:27:18 +0100 (Thu, 28 Jan 2016) $