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 "unstable", Prism will identify any parameters that are unstable and write "(Very wide)" in place of the confidence intervals for those parameters.

If you choose "ambiguous", if the entire fit is ambiguous, all confidence intervals will be replaced with "(Very wide)".