Anything that can change over time is potentially a timeline aware rule. To introduce the concept, here is an everyday example of data which can vary over time.
A person's total income will tend to vary over time as the person receives pay rises or moves between employments. Given that a person's income at a point in time can be represented as a Number, then a person's varying income over time can be represented as a timeline of Numbers.
For more information on identifying timeline aware rules see the Handling Data That Changes Over Time section of the Cúram Express Rules Reference Manual.
It is the job of a technical user to make a rule timeline aware. A rule diagram needs to be viewed in the technical view in order for timeline information and menu options to be available in the editor.