The short explanation
Stereoscopic 3D needs a separate image for each eye. HDMI defines several ways to carry both: frame packing (stacked, full resolution each), side-by-side and top-and-bottom (half resolution each). The display separates them again.
Where you run into it
3D televisions are gone, but 3D projectors and a large library of 3D discs are not, and that audience is very much still active. In such a chain every device has to pass the format through intact — a device that does not understand 3D framing can break it even while passing ordinary video perfectly.
How the DR HDMI 8K handles it
The DR HDMI 8K lists 3D ready among its capabilities, with the ability to pass 3D stereoscopic signal formats through. In a projector chain that also means the EDID controls apply as usual: if 3D is not being offered by the player, the capability data reaching it is the first thing to check.
Want to test your own HDMI chain?
The DR HDMI 8K is sold through the HDFury.eu web shop.