A typical error of reading is not a simaple figure, it's a function of Tobj (object temperature) & Tamb (ambient temperature)
Because Infrared Thermoemter is a "Relative" measurement instrument. (It compare the Tobj against Tamb)
So, the error has a minimum value when Tobj ~ Tamb,
that is when you measure something close to ambient temperature.
For ZyTemp, this minimum value (dTabs) = 0.15°C / 0.27°F ,
usually, our competitor is larger than 0.2°C / 0.36°F

At higher or lower temperature, there is a Gain Error , usually 2%.

Q: since the spec is "+/-2°C or 2%", so IRT is most accurate at Tobj=0°C?
A: No, as described as above, IRT use a "Relative" measurement, so IRT is most accurate at Tobj=Tamb,
In other word, when you pointing the IRT at a "near room temp" object, it will be very accurate.

Q: I can calibrate my IRT, by point it to a wall (25°C)?
A: No, even the Gain run away, the IRT still will show 25°C, you have to use a "Temperature Different From Tamb",
for example 0°C or 50°C
0°C is the most available temperature "standard", for ordinary user.