This one I actually know -- biohazards depend on more than just wattage -- spot size, wavelength, pulse characteristics, etc, all go into it. I think you really compare them based on the Poynting vector. Disclosure: I'm a GF backer and am a card carrying physicist who had this question on his qualifying exam in grad school. https://www.rli.com/resources/articles/classification.aspx

(ceo/cofounder here) You're right - it's the passthrough. CDRH is very strict about what could count as human access. We have some clever interlocks in place around the passthrough but if an examiner can place a mirror in the interior and measure laser output above the threshold without triggering the interlock, you're Class IV.

You can get Class I by preventing human access, I'm guessing the Pro can't have the same interlocks for some design reason.

I think the pro is 'feed-able', wonder if that's it.

Yeah, I just saw that on their youtube video; the sides can remain open during operation to allow etching/cutting of materials larger than the enclosure. That means human access is possible, so therefore the class IV.