It is probably explained in the documentation, like these screen shots:
Your mileage may vary, depending on device/brand/model.
PV clamping option is used for example to clamp PV to the PV extended high limit in case of aignal going over 20 mA. This mainly happens for levels.
If the tank is filled more than 100% then dp transmitter generates more than 20 mA. The PV high for level is 100%, and imagine you have set PV extended high to 102% which means signal till 20.32 mA is valid. Then if you have enabled PV Clamp, the signal more than 20.3 mA will be clamped to the 102% otherwise PV value will show you BadPv. In Honeywell it shows NAN.
Then in case of not PV clamping you don't know if the transmitter is faulty or you have encountered with over level. Also it would be the case for signals below extended low limit.
Hope to be useful.