SIwave fits a state space model to S-parameter elements so that it can interpolate and extrapolate their frequency response. This may become necessary if the user sets up a frequency sweep analysis that goes beyond the part’s data range. It tries for a maximum fitting error of 0.01, and issues a warning if the error exceeds that figure. If the final error is just slightly above the maximum (i.e. 0.02 or something like that), it will not cause any problem. When the error is higher, the fitted response will deviate a bit from the original S-parameter data for the part, but usually that will not have a dramatic effect on the results for the entire board.