There is not a flag that will enable or disable “Volumes” as a whole. The reason being is that there are different types of volumes such as Physics, Lightmass Importance, Cull Distance, and Post Processing to name a few.
The function is working properly when making the choices provided. There are a couple of things to note that may or may not have been clear in the documentation.
When using the ShowFlag.“object type” (ie. ShowFlag.PostProcessing) you will need to use a 0 to disable and a 1 to enable the volume or object you are calling show flag on. The show flag is a simple toggle for 0 = false or 1 = true to whether it should be on in the viewport.
Fore Example: ShowFlag.PostProcessing 0 to disable the post processing effects.
Another thing to note as well is that this does not turn on the outline of where the bounds of the volume are located. This will only enable or disable the object you select from the list.
No problem! Glad that worked out for you. There are improvements that need to be made to the documentation for some areas and we are definitely aware and making sure it’s on our list to get completed. If you ever come across anything that you feel could be improved in the documentation or are not sure about you can post in our Feedback for Epic section of the Forums. We do review this information and pass it along to our documentation team.