http://terminology.hl7.org/CodeSystem/v2-0290|2.0.0

mimeBase64EncodingCharacters

HL7-defined code system of concepts specifying the characters used in the MIME Base 64 encoding mechanism. These are for reference only, as they are not carried as explicit code values in any HL7 coded field. This table was published only in releases 2.3, 2.3.1, and 2.4 of the Standard, and was removed thereafter.