Cardinalities are assigned at the data class level but not at the level of the individual data elements. How should implementers of the data model understand the need for specific data elements? For example, if a system is designed to capture Employment status, is a status code absolutely required for a given period of employment? Is the start date absolutely required or is it permissible to have a code but not a start date. 

If cardinalities aren't assigned at the data element level, please provide guidance on how systems should implement data elements in general (can they ever assume any given data element will or won't be valued)