Hi justynb,
That's a snapshot of the ecad.io solution that Rachael mentioned earlier. It's not native to EAGLE, but as our integration with Fusion360 gets closer to being complete, EAGLE will begin to understand some 3D data.
Our website isn't meant to be the source for EAGLE's documentation. The Manual and built-in help are, however I agree that this feature overview could use some more information.
I'll let our website manager know, to see if we can add some details.
Best Regards,

Jorge Garcia
Product Support Specialist for Fusion 360 and EAGLE
Kudos are much appreciated if the information I have shared is helpful to you and/or others.
Did this resolve your issue? Please accept it "As a Solution" so others may benefit from it.