When a command has the letter i as a suffix, it means that the portion of the mesh to be applied is identified using an "index progression".
An index progression has the advantage of selecting more that a single region. So, for example, the index progression
2 3 0 5 6;1 2;2 3 0 5 6;
selects 4 regions (note "0" means and) which are:
2 1 2 3 2 3
2 1 5 3 2 6
5 1 2 6 2 3
5 1 5 6 2 6
In each region, an interval in each of the three index directions is selected. But when you use a minus sign, you are indicating a constant (i.e. not an interval). For example,
2 3 0 5 6;-2;2 3 0 5 6;
selects 4 regions, all of which are j-faces (i.e. j is a constant 2). This selects
2 2 2 3 2 3
2 2 5 3 2 6
5 2 2 6 2 3
5 2 5 6 2 6
There is a more detailed discussion in the manual. Both regions and index progressions
are closely coupled to selections using the index bars in the computational mesh.
