The annotations feature allows you to add a note which shows in your reports, which can help you keep accurate track of exactly when a promotion, campaign or collaboration starts/ends. Using annotations can help you measure performance increases / decreases such as clicks, sale value, commission and conversion rate.
To add an annotation to your account, simply click on the Annotations + button on the right-hand side of your account, this appears on the homepage and in the reporting menu area. Here you are presented with an option to add a New Annotation to your account. You may now enter the date and name of the annotation you wish to record against your account, and click Add.
For advertisers, you also have the option of adding annotation when you add a new creative to your account, so new banners, coupons and promotions. These will then appear in your reporting area once those creatives have started and ended.