If when you read the name on the bench you see the Pentagon in the background, that means the person was killed inside the Pentagon. If you see the sky then that person was on the plane that hit the Pentagon.
Here's a name with a ribbon from American Airways. It looks like they put these on those who worked for AA that died in the plane that hit the Pentagon.
The notes that loved ones left were very sweet and reminded me that these benches represent moms and dads, brothers and sisters...
The Pentagon Memorial is very touching. I highly recommend checking it out.