## Code Systems
### Terminology Usage
The ISO/IEEE 11073-10101 Nomenclature standard (11073 MDC) provides a comprehensive terminology for device models according to the 11073-10201 DIM. It's the preferred code system for profiles in this Implementation Guide. Reference identifiers, terminology codes, and descriptions are available at the [Rosetta Terminology Mapping Management System](https://rtmms.nist.gov/rtmms/index.htm) (RTMMS).

The PoCD profiles define rules for terminology usage:
* `Device.type` and `DeviceComponent.type` shall be present and have a coding from the 11073 MDC device nomenclature. This should ensure consistent coding using the ISO/IEEE 11073-10101 Nomenclature standard, which provides MDS, VMD, and Channel codes for device models according to the 11073-10201 DIM. As these elements are of type CodeableConcept there can be additional coding from other code systems.
* Same for the `DeviceMetric.type`, which shall have a coding from the 11073 MDC metrics nomenclature.
* `DeviceMetric.unit` is an optional element of type CodeableConcept. If present, it shall have a coding from the 11073 MDC units of measurement. Additional other coding is allowed as well. This element is the place for the 11073 MDC unit code, because the Observation resource has other constraints - see below.

The [Vital Signs Profile](http://hl7.org/fhir/observation-vitalsigns.html), which is part of FHIR core specification, applies for some commonly used measurement observations (e. g., heart rate, blood pressure, respiratory rate, oxygen saturation, body temperature):
* The PoCD profiles require an 11073 MDC coding in `Observation.code` and, for compound metrics, `Observation.component.code`. Depending on the type of measurement, the Vital Signs profiles may require a [LOINC](http://hl7.org/fhir/loinc.html) code. Note that the MDC code can be more specific than the Vital Signs LOINC code.
* The Quantity data type in `Observation.value[x]` and `Observation.component.value[x]` contains the units of measure. There is only a single coding that needs to be [UCUM](http://hl7.org/fhir/ucum.html) for all vital signs. Therefore the PoCD profiles require UCUM coding for all numerics.