Haha, I asked this a little while ago... basically, they're standards for testing your bottles against - in the US the standard is DOT (Department of Transportation) and in the UK the standard is HSE (Health and Safety Executive).
For some reason, the two organisations don't quite see eye-to-eye, which means that bottles tested to the US DOT standards cannot be used in the UK if you are a UK resident...
Apparently HSE is a more stringent test, but DOT bottles are made to a higher specification...
I think I got that right... I just regurgitated it...