Note: The class description below has been excerpted from the Hl7 2.4 documentation.
Sectional references made below also refer to the same documentation.
The value of such a field follows the formatting rules for an ST field except
that it is drawn from a table of legal values. There shall be an HL7 table number
associated with ID data types. An example of an ID field is OBR-25-result status.
This data type should be used only for HL7 tables (see Section 2.7.6, "Table").
The reverse is not true, since in some circumstances it is more appropriate to use
the CE data type for HL7 tables.
Note: validation of the input string value will only be initiated if the
corresponding HL7 tableid has been set. Basically if the object is created
without a tableid and the table id is not subsequently set, then then the input string value
will be subjected to a length check (<= 200 chars) and then stored into the object.