When building with sysbuild, an application uses a build configuration with one or more images or domains.
You can create such configurations using the same setup steps as for setting up a single build configuration, but with the sysbuild option enabled. Once built, the images and domains appear as children of the selected sysbuild configuration. The active image (context) is indicated by the blue color of its icon in the treeview. At any given time, you can only have one image or domain (and thus only one application) active.
For more information about sysbuild domains, see sysbuild domains in the Zephyr documentation.