Class A & B in Temperature Sensor (PT 100 & PT 1000)

These are accuracy classes. From Omega Engineering:

"There are four "Accuracy Classes" specified in IEC 60751, listed from least accurate to most accurate they are:

(Note: Accuracy Values are in Degrees C)

Class C = ±(0.60 + 0.01* t) (-50 to 500°C)

Class B = ±(0.30 + 0.005* t) (-50 to 500°C)

Class A = ±(0.15 + 0.002* t) (-30 to 300°C)

Class 1⁄3 DIN† = ±(0.10 + 0.0017* t) (0 to 150°C)

where t = temperature of interest in °C without regard to sign."
___

Pt100 RTDs have a resistance of 100Ω at standard temperature (0°C IIRC); Pt1000 RTDs are nominally 1000Ω, which makes them more sensitive. There are also two coefficients (0.385 and 0.392 Ω/°); I think they depend on slight differences in alloy. The Omega Engineering catalog has further information on this.