Note that your thorn is also a friend of all your thorns friend's friends. The thorn's variables are collected into groups. This is not only for convenience, but for collecting like variables together. Storage assignment, communication assignment and ghostzone synchronization takes place for groups only. A group name is compulsory.
TYPE designates the kind of variables held by the group.
This should be a comma-separated list of parameter names. It consists of a string made up of a combination DIM of the letters M , C , P depending on whether the layout in that direction is on the M inus face, C entre, or P lus face of the cell in that dimension. The block following the group declaration line contains a list of variables contained in the group. All variables in a group have the same data type, variable type and dimension. The list can be separated by spaces, commas, or new lines.