Background The commenter is referencing this section of the current published version of US Core version 4.0.0:     ValueSet: Birth Sex ...  __  Logical Definition (CLD) This value set includes codes based on the following rules: Include these codes as defined in  http://terminology.hl7.org/CodeSystem/v3-AdministrativeGender Code Display Definition F Female Female M Male Male Include these codes as defined in  http://terminology.hl7.org/CodeSystem/v3-NullFlavor Code Display Definition UNK Unknown * Description: *A proper value is applicable, but not known.  __   * Usage Notes *: This means the actual value is not known. If the only thing that is unknown is how to properly express the value in the necessary constraints (value set, datatype, etc.), then the OTH or UNC flavor should be used. No properties should be included for a datatype with this property unless:  __   1. Those properties themselves directly translate to a semantic of "unknown". (E.g. a local code sent as a translation that conveys 'unknown') 2. Those properties further qualify the nature of what is unknown. (E.g. specifying a use code of "H" and a URL prefix of "tel:" to convey that it is the home phone number that is unknown.)   As a result of implementer change requests, the Jan 2022 ballot version of US Core (version 4.1.0)Birth Sex Value Set includes the the concepts ASKU , and UNK  and can be directly viewed by following the links to VSAC:   ValueSet: Birth Sex The publication tooling does not currently support generation of this Value Set expansion. We anticipate updates to enable this feature before this guide is published. For this ballot, we have provided the manual expansion below for your review. Code System Display Definition F _ http://terminology.hl7.org/CodeSystem/v3-AdministrativeGender_ Female - M _ http://terminology.hl7.org/CodeSystem/v3-AdministrativeGender_ Male - ASKU _ http://terminology.hl7.org/CodeSystem/v3-NullFlavor_ asked but unknown - OTH _ http://terminology.hl7.org/CodeSystem/v3-NullFlavor_ other - UNK _ http://terminology.hl7.org/CodeSystem/v3-NullFlavor_ unknown - Logical Definition (CLD) This value set includes codes based on the following rules: Import all the codes that are contained in  http://cts.nlm.nih.gov/fhir/ValueSet/2.16.840.1.113762.1.4.1 Import all the codes that are contained in http://cts.nlm.nih.gov/fhir/ValueSet/2.16.840.1.113762.1.4.1021.103