What does

+@foo_hosts username

mean in a user's .rhosts file?

I presume it is defining a group of hosts in some way, but I can't figure out where to look for the definition of @foo_hosts. I found the name foo_hosts mentioned in the output of ypcat netgroup.byhost but not in a way that seems to associate it with any list of hosts.

uname reports SunOS 5.8