Selector number of a paquet's data

Hi,
I am using Lonscanner to capture the packets on a Lonworks network and i'm trying to analyse the data.
If we take the example in section 4. Example Packet Logs (page 55)
http://www.echelon.com/assets/bltfd43556611020f6b/078-0305-01B_LonScanner_FX_Users_Guide.pdf
we have Data: NVI_848 = 51201
which means that the network variable is an input network variable with selector number 848.
How can i know the signification of the selector number in the paquets and how did they assume that it is an SNVT_switch in the example?

Thank you.