I don't get SignC or SignF and how you can equate to 'Positive' or 'Negative', I couldn't find a reference in the PBP manual.
Explanation would be appreciated
For positive temperatures the output increases from 0000h as the temperature increases from 0 deg C.
For negative temperatures the output decreases from FFFFh as the temperature falls below -0.5 deg C.

To check for negative any output bit can be checked for 1, when that bit is outside the temperature range of the sensor. The data sheet says

The sign bit (S) indicates if the value is positive or negative: for positive numbers S = 0 and for negative numbers S = 1.

In the program RawTemp.8 is chosen as the sign bit.