Like many insults hurled in Overwatch 2’s in-game text or voice chat, this one can be quite the compliment, as well. “Diff” just means “difference” or “differential” and it speaks to a clear chasm between roles or characters. If each team has a Mercy, but one is significantly out-healing the other, then there’s a Mercy diff. If one tank is putting out low damage, little mitigation, and has a fair amount of deaths, then there’s a tank diff. So, if you’re playing tank and doing well and someone says “tank diff” then you know your team is proud of you, but if you’re struggling and someone on your team says it, you know they’re not mad, they’re just disappointed.