The criteria of a rule can be made complex by configuring it with two or more parts.
In complex criteria, each of the parts has a property with operator and operand. The parts are linked by AND or OR.
If you configure criteria with three or more parts and use both AND and OR between them, you also need to use parentheses to indicate how the parts are logically connected. For example, (a AND b) OR c differs in meaning from a AND (b OR c).
When you add a third criteria part on the user interface, lowercase letters appear before the parts and an additional field is inserted at the bottom of the configuration window.
The field displays your criteria parts in short, for example, a AND b OR c. You can then type parentheses in the field as needed.