Atomic Data TypesAtomic data types cannot be broken down into smaller units. These data types can be primitive or derived. For example, the string primitive data type cannot be broken down further into smaller units, such as a characters.
List Data TypesList data types are derived data types that contain a set of values of atomic data types elements referring to a list data type can contain a value only from that defined set. For example, you can create a list data type called pointlist, which is derived from the data type decimal and contains the following values .5, 2.5, and .75. Any element that is associated with the pointlist data type can contain only .5, 2.5, or .75 as its value.
Union Data TypesUnion data types are derived from the atomic list data types. For example, if zip name is an atomic data type that can n create a data type called zips that can contain a combination of both these data types. Now, if you create an element called ZIPCODE of type zips, then ZIPCODE element can contain the values shown in the following code snippet:
<ZIPCODE>90547 96098 90contain only string values and zipnum is a list data type that can contain a list of zip code numbers, then you ca878 </ZIPCODE>