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 a ST field except that it is
drawn from a site-defined (or user-defined) table of legal values. There shall be an HL7
table number associated with IS data types. An example of an IS field is the Event reason
code defined in Section 18.104.22.168, "Event reason code." This data type should be used only for
user-defined 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 user-defined tables.