You have a choice, in the Confidence tab, to either mark selected parameters as "unstable", to mark entire fits as "ambiguous", or neither. Beginning with Prism 9.0, the option to mark parameters as "unstable" is the default option.

If you choose the older approach to identify ambiguous fits, Prism places a tilde ("~") in front of the parameters that have super high dependencies. The parameter value is what is used to calculate the Y values for the fit curve. But with super high (>0.9999) dependency, you could get a curve that fits just as well with very different parameter values. These values that are preceded with tildes are really not useful.