There is a field in Income, for 'Annual variable pay'. How should this be used?

This field has been added for your clients who have a fixed annual salary and also, receive a variable bonus or work paid overtime. In this example, the client receives a salary of £55,000pa and she received variable bonuses of £5,000 last year:

It is not a mandatory field, and if completed, when viewing the plan overview/comparison all salary will show as one colour.