Making a field required is one of the most frequently used validation methods. By selecting Required in the Properties of a field, you specify that users will not be able to submit their entry unless they fill the field in.
You also need to enter a validation message that will be displayed to users who attempt to go to the next page without filling the field in.
However, you may want the field to be required only in specific situations. To make it work that way, select the Only when… option under Required, and—apart from entering the validation message—build a condition to specify when the field is required. For example, it is easy to create a Maiden Name field which is only required if Female has been selected in the Gender field.
The conditional requirement mechanism is used when a field is supposed to be visible and enabled all the time, and only the requirement status changes. If a field is sometimes (i.e. under a certain condition) hidden or disabled, it is no longer required.