In order to create a new validation rule, select the 'Rules' tab from within the Schema that you would like to apply the rule to and select the 'Add assignment rule' button.
This will bring up the 'edit assignment rule' dialog, where a unique name can be specified and the attribute which should be modified can be selected in the 'assigned attribute' field.
A condition can then be added to state 'if [Attribute X] (condition) [value]', then the 'assigned attribute' is assigned the value described in the 'calculated value' field. The calculated value can be another attribute, a string of text, a numerical value, date/time, or an expression, as with validation rules.
Additional if/then clauses can be added to the assignment rule so that the assigned attribute takes on a different value depending on the conditions specified within an if/then clause (as per the example). These additional if/then clauses can be added through the 'Add Another If/Then' button and will require another condition and calculated value to be set.