This case-sensitive code system http://hl7.org/fhir/us/cdmh/CodeSystem/pcornet-body-position-codes defines the following codes:

Code	Display	Definition
01	Sitting	Vital Sign was measured in a sitting position.
02	Standing	Vital Sign was measured in a standing position.
03	Supine	Vital Sign was measured in a supine position.
NI	No Information	There is no information about the position during Vital Sign measurement.
UN	Unknown	Body position is unknown during measurement.
OT	Other	Body Position is Other.