How does nessus know exactly which application is running

[alan donald <netquantum_at_yahoo.com>]

 (This question is somewhat related to one my earleir
 posts.)
 
 I am a bit confused. I dont seem to understand how
 nessus is knowing which application is running.
 Since
 it has to know this information in order to launch
 an
 exploit. 
 
 Is it just matching a banner
 OR
 does it have a database of responses for each
 application like maybe nmap does. 
 OR
 It is fingerprinting the application in some other
 way.
 
 


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