The use of a list aggregate between common_datum_list and datum_reference_element imposes an ordering on the elements included in the reference compartment. At each node of the tree structure a set of modifiers may be applied to identify the particular operator required. The modifiers are separated into two extensible ENUMERATION types depending on whether additional values are provided. The simple_datum_reference_modifier is used when there is no additional value provided and the datum_reference_modifier_type is used when there is a value provided and provides an assertion that the shape of the associated feature used to establish the datum is circular, cylindrical, spherical, two parallel planes, or a projected length as defined in ISO 5459.