The Mac operating system comes with a single desktop space by default, but users can set up multiple desktop spaces, identified as Desktop 1, Desktop 2, and so on. All desktop spaces are accessible from the Mission Control icon on the Dock. You can choose to specify which of the desktops (or all) each application opens in. This feature is useful for people who use multiple spaces for specific purposes. For example, a desktop that is primarily used for working with correspondence might have Mail, Contacts, and Reminders open. Perhaps a space for working with photos would be home to Photoshop, Aperture, or Apple's Photos app.

How you organize and use your desktop spaces is up to you, but as you work with the desktops in Mission Control, you'll likely come across apps that you want open in all of your active spaces. You can set apps to open in all spaces, so that when you switch between desktops, the same apps are available across all of them, in addition to the apps you've assigned to specific desktops.

Before you can assign an app to a space, you first need to set up multiple desktop spaces. You can do this using Mission Control. To add multiple desktop spaces to your Mac:

Click the Mission Control icon in the Dock to open the space bar at the top of your Mac screen.