It defines if the data trasfer (in case of a remote capture) has to be done with UDP protocol.
If it is '1' if you want a UDP data connection, '0' if you want a TCP data connection; control connection is always TCP-based. A UDP connection is much lighter, but it does not guarantee that all the captured packets arrive to the client workstation. Moreover, it could be harmful in case of network congestion. This flag is meaningless if the source is not a remote interface. In that case, it is simply ignored.