Look at the back of most full-face helmets sold in the U.S. today and you’re likely to see a SNELL logo sticker. It means the helmet meets or exceeds the Snell Memorial Foundation’s standards for motorcycle helmets. Here’s a snippet from their (terribly outdated) web site, www.smf.org:
In order to continuously monitor the quality of helmets being sold to the public, Snell purchases and tests samples of currently certified helmets from the marketplace. These helmets are tested only in Snell labs by Snell technicians. Should a currently certified helmet fail, the helmet manufacturer must take corrective action to Snell’s satisfaction.
In other words, if you make a helmet and want to have that Snell logo on the back so you can tell potential buyers that your helmet is Snell certified (and presumably safer than one that isn’t), your helmet has to pass Snell’s tests. In the American motorcycling community, seeing the Snell logo on the back of a helmet is generally accepted as a good thing.