The value of _map_segment.mask_array_id, if given, describes the array structure into which the mask for the map is organized. If no value is given, then all elements of the map are valid. If a value is given, then only elements of the map for which the corresponding element of the mask is non-zero are valid. The value of _map_segment.mask_array_id differs from the value of _map_segment.array_id in order to permit the mask to be given as, say, unsigned 8-bit integers, while the map is given as a data type with more range. However, the two array structures must be aligned, using the same axes in the same order with the same displacements and increments. This item is a pointer to _array_structure.id in the ARRAY_STRUCTURE category.