An annotation is an element that can be used to put comments in a flow.
Currently you warn users of unpaid orders with a pop-up message in the client. Later you want to extend this warning with an e-mail message send to the user. To remember this you can use an annotation and put it beside the current activity that warns the user.
2 Common Properties
For details, see Common Properties.
3 Annotation Flow
An annotation flow is a connection that can be used to link an annotation to a flow object(s).
For example, this is an annotation flow linking an annotation and a microflow call activity: