@TT = +75C is the terminal temperature. It is in the "maximum" section, so the diode is rated for max 5A when the terminal temperature is +75C.
Vf @5A according to table 2 is about 1.05V @25C. Power dissipation = 1.05V * 5A = 5.25W
Temp rise = 10 * 5.25 = 52.5C
Junction temp = +75C + 52.5C = 127.5C but it will be less since the forward voltage decreases with increasing temp.
Approximate with -2mV/C so Vf @5A @+125C is about 0.85V.
Power dissipation = 0.85V * 5A = 4.25W
Temp rise = 10 * 4.25 = 42.5C
Junction temp = +75C + 42.5C = 117.5C but it will be a little higher since the temp is less than 125C, so Vf is a little higher than 0.85V.
Expect the junction temperature to be about +120C when the terminal temperature is +75C and the average current is 5A.