How is this virtual network boundary created?

[svenakela]

In this example image, there is a dotted boundary with a virtual network icon. I can't find any source for the diagram and would appreciate if someone could fill in how that boundary is defined.

[Potherca]

After all preprocessing (includes etc), PlantUML saves the diagram's source code in the generated PNG Metadata in the form of encoded text. ¹

So, if you have the original image, it might be possible to see the used source:

PlantUML saves the diagram's source code in the generated PNG Metadata in the form of encoded text. So it is possible to retrieve this source by using the query parameter metadata, giving it some image URL.
For example, if you want to retrieve the diagram source of the image http://i.stack.imgur.com/HJvKF.png use the following server request: http://www.plantuml.com/plantuml/?metadata=http://i.stack.imgur.com/HJvKF.png. ²

But, to be honest, the diagram looks like it might not have been made with PlantUML.

Footnotes

1. https://plantuml.com/command-line#ce21470ab49d1d19 ↩

2. https://plantuml.com/server#ce21470ab49d1d19 ↩