Some lightweight disk rotors place too much stress on 6-bolt disk mounts. Any disk without a continuous ring of material linking the bolt holes will load each bolt individually rather than spreading forces around all 6 bolts, causing bolt loosening and sometimes total failure of the disk mount.
The American Classic disc reinforcing rings bolt on to the outside of the rotor, spreading the load and eliminating this problem. Sold as a pair, weight is 6g each.
American Classic say:
"There is a problem with some disc rotors on the market. The rotors are cut out in the bolt area and do not have a reinforced center to properly support the international 6 bolt mounting system. The cut out rotors are weak at the bolts and result in failures. Bill Shook engineered the disc reinforcing ring to support the 6 mounting positions, effectively tying them together to work as a unit. Can be used on other hub brands."