message type and data types
the following is a statement in help.sap
"For technical reasons, a data type is not sufficient to describe the instance of a message. In XML schema, data types are defined as abstract types that are not yet fixed to an element"
as per the above where can i see the data types being defined as abstract types ? can i see them in the xsd ?or did i misunderstand the statement ...
"that(data types) are not yet fixed to an element"
what does it mean? why message types alone can be assigned to elements , why not data types?
if message types alone has some functionality why cant data types have them ?
please elaborate .