The number given refers to a specific grid required for formulating differential operators. The grid may consist of a centre and an arbitrary surrounding polygon. As model variables may be defined on vertices of the polygons or in the middle of a polygon edge, this generates some different grid descriptions, because each of those is defining their own centre and surrounding polygon. Each of these dependent grids needs their own set of centre longitude/latitude The number is associated with an attribute of the respective grid description which consists of the centre longitude/latitude and the longitude/latitude of the boundary polygon vertices.
The following picture shows a triangle as base, a hexagon around the triangle's vertices and a quadrilateral around the edge midpoints.
(a). Triangle (i) (pressure, temperature, ...)
(b). Quads (l) (wind velocity ..)
(c). Hexagons respectively pentagons (v) (Vorticity,...)