Dunnett's method compares the treatment means with the mean of a control level. The control level is usually a standard against which you want to compare all other levels. It produces intervals for the mean differences which have the simultaneous confidence level that you specify.

The simultaneous confidence level is particularly important when you perform multiple comparisons. For any given individual confidence level, the chance that at least one confidence interval does not contain the true difference increases with the number of comparisons. Consequently, the simultaneous confidence level is always lower than the individual confidence level for multiple comparisons.

The individual confidence level is the percentage of times that a single confidence interval includes the true difference between factor levels if the study were repeated multiple times.

The simultaneous confidence level is the percentage of times that a group of confidence intervals all include the true differences between factor levels if the study were repeated multiple times.