What is a subfield?

A component part of a field. Contains a defined data element.

Source:google.com.ng/search?hl=en&defl=en&q=define:Subfield&ei=-US3Soq4L9mf4AaIq4F9&sa=X&oi=glossary_definition&ct=title">http://www.google.com.ng/search?hl=en&defl=en&q=define:Subfield&ei=-US3Soq4L9mf4AaIq4F9&sa=X&oi=glossary_definition&ct=title