I’m interested in this too. Joe Friel calls this zone the “happy hard” zone that most recreational runners/cyclists spend most of their time. Especially when running in social groups. They feel like they accomplished something because they went “hard” but it really wasn’t hard enough to make substantial gains, yet certainly not easy enough to serve as recovery for other truly-hard days.
There are lots of proponents of this poloarized methodology (like 80/20 etc) with almost all of them having a middle zone that is generally avoided (with exceptions).