Listening socket backlog size. I can not understand the wording of a manpage

My man pages for kqueue give me the following:

 EVFILT_READ         Takes a descriptor as the identifier, and returns
                     whenever there is data available to read.  The
                     behavior of the filter is slightly different
                     depending on the descriptor type.

                     Sockets
                         Sockets which have previously been passed to
                         listen() return when there is an incoming
                         connection pending.  data contains the size of
                         the listen backlog.

The question follows, is above backlog value equals the number of total backlog size, or, the value equals the actual backlog size, as the event returns to a user?