Pretty sure if you look at /usr/include/sys/stat.h, what "ls" is showing above corresponds to the line
#define SF_RESTRICTED 0x00080000 /* restricted access */
which is one of the bits that can be in the st_flags member of the stat structure (a field that's not common to all Unix flavors!).
I don't have El Capitan beta installed, but that's in the include file on Yosemite, too.
On Yosemite, I can set and clear the flag as root (using chflags(1), or from C, chflags(2)). I haven't yet grabbed the latest Yosemite source, untarred it all, and grepped for SF_RESTRICTED, so I don't know what if anything it does prior to El Capitan.
sh-3.2# touch /tmp/testfile1
sh-3.2# ls -lO /tmp/testfile1
-rw-r--r-- 1 root wheel - 0 Sep 7 10:14 /tmp/testfile1
sh-3.2# chflags restricted /tmp/testfile1
sh-3.2# ls -lO /tmp/testfile1
-rw-r--r-- 1 root wheel restricted 0 Sep 7 10:14 /tmp/testfile1
sh-3.2# chflags 0 /tmp/testfile1
sh-3.2# ls -lO /tmp/testfile1
-rw-r--r-- 1 root wheel - 0 Sep 7 10:14 /tmp/testfile1