A text annotation is a comment in a diagram.
You can attach a comment to a specific element in the diagram, by choosing the text annotation icon above an element and placing it next to the element. You can also attach comments to a pool, independent of any specific element.
To do this, select the text annotation icon from the BPMN elements menu to the left of the screen, and drop it on the diagram inside a pool or lane outline.
Text annotations are unformatted text. To enter text, select the annotation and go to the Details panel, General tab, General pane. You can also enter text directly into the annotation when you first create it.