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1. A VD may have these properties
   1. An Administrative record
   2. A definition
   3. A data type
   4. A relationship to another VD
   5. A unit of measure, especially quantitative results
   6. A Format template e.g., CDASH's dates, ADaM's SAS format, etc.
   7. A length descriptor, for character data type
2. A VD may be either enumerated, described, or continuous
   
   1. Enumerated VD contains discrete values, e.g., {MILD, MODERATE, SEVERE}
   2. Described VD contains a description of what data values are permissible, e.g., LLT from MedDRA by MSSO
   3. Continuous VD contains awhose PV may be free text, with or without any nomenclature.
3. A PV may belong to 1..n VD
   1. A Permissible Value may have a Value Meaning
      1. Representation of its value meaning is independent of, and shall not be constrained by, the representation of its corresponding value
   2. A Value Meaning may have 1:n representations, hence taking a unique form of PV
   3. For a Permissible Value to become a member of VD, use a start date
   4. To retire a Permissible Value from a VD, use an end date

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