To allow data to be coded when only incomplete locational information is available, a series of special purpose codes has been created for each hierarchical level within the ASGC’s Main Structure and for SRs within the SR Structure. These codes are used when people provide limited address details or have no fixed place of abode.
Special purpose codes enable data to be coded to the broadest hierarchical level reported. For example, a reported address may be sufficiently detailed to allow it to be coded to a Capital City SD within a S/T, even though it cannot be coded to an SSD within that Capital City SD.
Coding data to a higher level unit in a hierarchical classification without also coding it to the lower levels would cause the data value for the higher unit to no longer equal the sum of data values for the lower units. Special purpose (i.e. dummy area) codes have therefore been created for each hierarchical level in the Main Structure and for SRs within the SR Structure, to which geographically undefined data can be classified.