Units for ppm?

I just found out that not everyone uses the same ppm units? I am a night person and my plant light is also on at night. Some of the peeps I communicate with in the middle of the night are not in North America.

We use ec times 500 and they use ec times 700. Wtf. Not sure how often this turns up but it does make talking about ppm interesting until you figure out what is going on.