I've never used a PTC, so I'm speculating, but in the absence of any other response, I'll take a shot at what I think the PTC sensor function is with the prescribed AI card.

It appears to me that although the card is an analog input card, the output for the PTC is not an analog value, ie, a temperature value.  Rather, the reported output for the PTC channel is the temperature "status" as indicated by bits in the status word.

The x-axis is not in specific time units, but shows the progress of a motor heating up and cooling down and status bit changes as that happens.

The Fig 4-2 diagram is is in Ohms.  I assume that standards governing PTCs have a table for resistance vs temperature, which typically very non-linear with sharp increase at points in the curve.   The diagram is showing that between 18 and 550 Ohms things are OK.  From 550 to 1650 Ohms, the Pre-warning bit is enabled, telling you that the motor is heating up.   Above 1650 Ohms, the 'response' bit is enabled, which I assume is the condition where one wants to shut the motor off to avoid damage from overheating.   As the motor cools the status changes to "Prewarning" as the resistance goes below 750 Ohms and down to 540 Ohms.  Below 540 Ohms, the motor is back to a normal operating temperature range.

For status word IB x+1,

- normal running temperature status word would be 00000000, 0x00
where LSB 0 = 0 Measured temperature below the response range
bit 2 = 0  Measured temperature outside the prewarning range

- 0x05 and 0x01 are alarm states indicating over heating

-0x04 is a warning of being close to an overtemperature condition.