This EditMode is used to create Sound Entities in the map. These are used to add ambience sound, static sounds like water flowing, or pretty much any sound you want.
To create a Sound Entity, just click on the grid when this EditMode is active. This will create an “empty” Sound Entity, meaning it is just a container. Optionally, you can set up the .snt file that will be used by the newly created Sound Entity, in the only input that shows on the EditMode window.
Have into account that these options will be valid for objects created right after changing them, so any Sound Entity that is already created will keep its settings.
More on sounds here.