David Abrams, Senior Calman Product Manager at Portrait Displays, explains:
“We’ve already set up our system for HDR and we’ve already done BT.2020 but the monitor has a second memory based on its color space flag. So remember, we were talking about the info frame flag that content sends to the display to say, ‘I’m 1,000 nits, 4,000 nits or 10,000 nits’. There’s also a colorimetry flag in the AVI info frame that tells you if the content is BT.709 or BT.2020. The LG OLED has two memories: if it sees BT.709, it goes into a BT.709 memory; if it sees BT.2020, it goes into a BT.2020 memory. What we’re going to do is tell the LG that we’re sending BT.709 HDR, which isn’t really a standard because BT.709 is an HDTV SDR standard. Therefore, we’re going to trick the display by sending P3-D65 content down a BT.709 flag and calibrate the BT.709 memory to P3. Getting a BT.709 flag going to a P3-D65 calibration, the generator is going to send a BT.709 flag, which triggers it into that P3-D65 mode. On many of today’s color grading systems, when you set up a P3-D65 workflow, the HDMI output of these professional cards will actually automatically send a BT.709 flag. So, if you’re a colorist using something like DaVinci Resolve with a Blackmagic 4K output card or a 4K UltraStudio Mini, you’ll be able to set up your timeline for P3-D65 and the output of the card will automatically send a BT.709 flag, which will automatically put the display into P3-D65. If you set up your timeline and your workflow for BT.2020, the display will automatically go into BT.2020.”
The Daejeon Chronicles 
My Decklink card will send a flag for what is specified, 709, 2020 or P3. For HDR I send P3 inside a 2020 container. Sending P3 flagged as 709 has drawbacks because the volume is more than the container.