A stack of values (incl. representation) associated with a given grid cell.
Note that it is possible for a cell to NOT have any values associated with it. This is a consequence of the rigidity of the raster grid definition as per ISO; i.e., a grid may be made to cover a field, but some parts of the grid may not overlap the field. e.g., the grid is perfectly rectangular; if the field is, say, trapezoidal, some grid cells will not overlay the field.