I'm trying to create a diagram like in the image. However I don't know how I can use a shape like the "msg" field. In other diagram software it's called "document". Is it possible to use this shape in an Activity diagram?
Unfortunately this shape does not yet exist on PlantUML shape.You can make a wanted feature...
See doc here:
It does exist only on Ditaa shape:
| msg |
If that can help,Regards,Th.