A text annotation is a comment that can be added to a diagram.
Text annotations can be added to :
A specific element by choosing the text annotation icon above an element and placing it next to the element
A pool by selecting the text annotation icon from the BPMN elements menu on the left of the screen, and dropping it on the diagram inside a pool or lane outline.
Text annotations are unformatted text. You can enter text directly into the annotation when you first create it. To modify the text of an annotation, select it and go to the Details panel, General tab, General pane.
Text annotations are useful to exchange information with other business analysts viewing or developing the process. They are also a useful way of storing information that might be required for process maintenance (similarly to comments in .md source files).