I don't know but I'll guess. You should confirm with your retailer. It's probably maps, and an HRM. Garmin sells both separately, and I think the 810 comes with a base map that's not terribly useful unless you buy their more-detailed ones.

I think their latest HRM goes for about $70 and you can put free, high-quality Open Street Maps on the 810, I've been using them on my 800 for years.