If defaults is used then update-rc.d will make links to start the ser-
vice in runlevels 2345 and stop the service in runlevels 016. By
default all the links will have sequence code 20, but this can be over-
ridden by supplying one or two NN arguments; one argument overrides the
sequence code for both start and stop links whereas of two arguments
the first overrides the code for start links and the second that for
stop links.
Я никогда не использовала просто defaults, поэтому и цитирую вам man update-rc.d,
который вы, собственно, и сами можете прочитать.