<xsd:documentation>Since various activities may use different points as
measurement datums, it is useful to characterize the point based on its usage.
A well reference datum may have more than one such characterization.
For example, it may be the datum used by the driller and logger for measuring
their depths. Example usage values would be 'permanent','driller', 'logger'
'WRP' (well reference point) and 'SRP' (site reference point).</xsd:documentation>