Hi,
The livetri.zip file is the master catalog file of all the products of Symantec that can be updated via liveupdate. it also contains the latest revision number of the definitions for those products.
So, if a client is doing a liveupdate session in a frequent manner, its possible that the new revisions for SAV or any other product have not been released by Symantec yet. So, the livetri.zip file remains the same.
Thats the reason the clients can get the return code 304.
Note: LiveUpdate clients that connect to Symantec servers do not use the Livetri.zip file.
I hope that answers your question.
Aniket