Principal Diagnosis in the US (CMS) has a very specific meaning:
The use of “Diagnosis, Active (ordinality: Principal)” has a specific meaning in the 2014 measures for hospitals and should be consistent with its definition in the Uniform Hospital Discharge Data Set (UHDDS) as “that condition established after study to be chiefly responsible for occasioning the admission of the patient to the hospital for care.” The designation refers to the Principal Diagnosis of an episode of care, as in the previous definition. This is typically determined at or after discharge time by a coder and is used for the billing transaction. In EHRs, the Principal Diagnosis is typically chosen from among the diagnoses that were active during the encounter and, if consistent with the UHDDS definition, should be labeled as “Principal.” For the purpose of measure computation, the principal diagnosis should be considered to start during the episode of care.

This is also true of Principal Procedure:
Inpatient hospital claims require reporting the principal procedure if a significant procedure occurred during the hospitalization. The principal procedure is the procedure performed for definitive treatment rather than for diagnostic or exploratory purposes, or which was necessary to take care of a complication. It is also the procedure most closely related to the principal diagnosis. The provider enters the ICD-9-CM code for the inpatient principal procedure on the Form CMS-1450 FL 80 titled Principal Procedure Code and Date. This includes incision, excision, amputation, introduction, repair, destructions, endoscopy, suture, and manipulation.

Currently the SNOMED CT qualifier concept "Principal" (63161005) is used in the value set Principal (2.16.840.1.113883.3.117.1.7.1.14) . This is used as an Ordinality Attribute. This is used to identify when a disease, procedure (sometimes a symptom?) are used as specified above. The problem is that this phrase can also be used to describe the condition of clinical focus during a hospitalization. This is not necessarily the same condition "tagged" as the "Principal Diagnosis" which is a billing use.

After much discussion it was clear that we need two different concepts, one for "principal" when used for billing, and another for "primary" (or principal in the non-billing sense) to represent the clinical focus. This was also discussed with Jim Case to clarify what might be added to the US SNOMED CT extension.

Our final proposed solution is:
1) Request the creation of two new contextual qualifier concepts in the US extension:
1.a) Principal billing diagnosis - this concept is to be used to identify conditions that are designated as “principal diagnosis” as defined above.
1.b) Principal billing procedure - this concept is to be used to identify conditions that are designated as “principal procedure” as defined above.
1.c) Both these concepts would be children of a new concept “”Billing diagnosis” that would be a sibling of the existing SCT concept “Principal diagnosis”. They are not children of the existing concept.
2) Give guidance that the existing “Principal diagnosis” concept (8319008) is to be used when identifying conditions considered “of primary focus” in the clinical treatment of the patient. This is consistent with the current synonyms in SCT and what appears to be the intended meaning.
3) Add “ Principal procedure” as a synonym to the existing “ Primary procedure” concept (399455000) to have these two concepts consistent even though the preferred name uses Principal in one place and primary in another.
4) Give guidance to use the general qualifier “Principal” (63161005) [synonym “primary”] if needed as an attribute to identify symptoms of a primary/principal focus.