This code system http://hl7.org/fhir/v2/0920 defines the following codes:

CodeDisplayComments
AAlternateThis is a specimen that is acceptable as a replacement for a preferred specimen. In the following field (OM4-17), the sequence number of the preferred specimen must be messaged.
PPreferredThis specimen is Preferred for all attributes (Container and Additive) identified in the OM4 segment.