The Date Modifier feature was added starting with rev. 46. It offers the possibility to interconnect multiple calendar fields so that the selection from one calendar can be set up to be the start or end date of another.
Let's assume that you have 2 calendar fields, Calendar A and Calendar B. To set up the date selected within the Calendar A field to be the minimum selectable date for Calendar B, please head to the following path:
Components >> RSForm!Pro >> Manage Forms >> edit your form >> Components >> edit the Calendar A field >> Validations tab
and set the Date Modifier drop-down to Selected date becomes 'Calendar A' minimum date.
You can find more information on the Date Modifier feature here.