Step 1.
    include "addAllRegionOptions.H"

    Adds the -allRegions, -regions and -region options to argList.

Step 2.
    include "getAllRegionOptions.H"

    Processes the options with -allRegions selecting everything
    from the regionProperties.

    OR use -regions to specify multiple regions (from
       regionProperties), and can also contain regular expressions

    OR use the -region option

    Specifying a single -regions NAME (not a regular expresssion)
    is the same as -region NAME and doesn't use regionProperties

    Creates a `wordList regionNames`

Step 3.
    Do something with the region names.
    Either directly, or quite commonly with the following

    include "createNamedMeshes.H"

    Creates a `PtrList<fvMesh> meshes`

STYLE: add description to some central include files