I'm trying to research many of the same questions. There doesn't seem to be many DIY-LED experts around lately. Take a peek at this site, this helped me:
https://www.rapidled.com/led-university-1/
I believe the second number relates to it's max. rated wattage. The second number seems to relate to drivers max. voltage. I know we multiple (current) amps x LED voltage to get max. wattage. If you plan to run this board at it's max 2800 mA If we take 54 / 49.86 VDC; you'd be utilizing 92% of the drivers potential give or take. You could probably run the board at a higher wattage without running your driver as hard with the HLG-150 but I'm not sure because the second number is the same. Are you looking to run this board at 100W, 120W, 150W?
Maybe one of the resident experts can lend their knowledge here.