What is the "nominal temperature threshold" (NTT) on a thermistor?
Yes, the datasheet shows seven different order codes on page 3. 

So there are seven different parts: 


B59801D0080A040 is the third one down in that list. 

As you wrote, the 80A says its \$T_{NTT}\$ is 80 degrees C. That is the one in your posted photo. 

It's resistance rises dramatically above 80C. 

It doesn't look like it is intended for precise measurement. If it were, I would expect the datasheet to give the temperature coefficient, i.e. the change in resistance for a change in temperature, and it doesn't. 

It is optimised for a 'trigger point', a dramatic change in resistance over quite a small range of temperature change. It's more like a thermostat, 'full-on' or 'full-off', than a sensor for actually measuring temperature.

The Spec says resistance changes from 570Ω at +/-5 degrees K to over 10,000Ω at +23 degrees K. A change of about 20x in resistance for a temperature change of about 6% (in Kelvin). 

It's operating temperature range is stated as -40C to \$T_{NTT}+23C\$ providing the voltage used to measure it's resistance is \$V_{measure}\leq 7.5C\$.

However, the graph for \$T_{NTT}\$ goes up to 160C.

Apropos nothing:
That operating point is somewhat unfortunate. If it triggered below 100C, say 90C, it'd be able to detect water is close enough to boiling, without actually boiling, and so is a good temperature for making tea, Which would save quite a lot of electricity. (I'm British :-).