Error Tolerance
Key Error Tolerance in SABR Model refers to the optional maximum error tolerance used by QuantLib in the construction of the volatility cube.
If the error associated with the swaption price fit exceeds the value here, then the calibration is considered failed.
Note there are two choices for defining the above error that can be set in Use Max Error
If absent, it is set by default equal to 0.0015 if Vega Weighted = TRUE and 0.01 otherwise.