In the example below:

<CONCEPT>
<DESCRIPTOR>Artificial Intelligence</DESCRIPTOR>
<SC>PRO</SC>
<BT>Intelligence</BT>
<BT>Software</BT>
<RT>Computer programming</RT>
<RT>Cybernetics</RT>
<NT>Neural networks</NT>
<USE>Research</USE>
<USE>Development</USE>
<UF>Machine learning</UF>
</CONCEPT>

Descriptor is the main term. This record will be used when the user enters the keywords "Artificial Intelligence".

BT is a broader term. This is used if the user performs a search which does not return any search results. The system will then say something like "there are no results for your keyword would you like to search for Intelligence, Software etc".

NT is a narrow term. We import this term but do not use it at the moment in our search engine.

RT is a related term. If the user searches for accessories, the system will also search for these terms ie. Computer programming and Cybernetics.

UF is a used for term. If the user searches for accessories, the system will also search for these terms ie. Machine learning

USE is a use this word term. If the user searches for accessories, the system will also search for these terms ie. Research, Development

SC - we don't use this term, it looks to be some sort of categorisation. Our import ignores it.

So, to summarize: when the user searches for Artificial Intelligence the search engine will search for Artificial Intelligence, Computer programming, Cybernetics, Machine learning, Research, Development.
If no results are found then the system will suggest that the user search for Intelligence or Software.