[CentOS] In /etc/auto.master, why "+auto.master"?

Wed Jan 24 23:38:35 UTC 2018
Robert Nichols <rnicholsNOSPAM at comcast.net>

Why is the line "+auto.master" present in the /etc/auto.master file? Does this not create a loop, and perhaps is the reason why, on every boot, I see the message, "automount[3260]: problem reading master map, maximum wait exceeded"?

This is in an unmodified /etc/auto.master from autofs-5.0.5-133.el6_9.x86_64 in CentOS 6.9. The only other non-comment lines in that file are:
    /misc   /etc/auto.misc
    /net    -hosts

Bob Nichols     "NOSPAM" is really part of my email address.
                 Do NOT delete it.