What's really happens when I create a "New Sprite Image" inside my Xcode images.xcassets?

Question

Days ago I asked for How check the .spriteatlas sizes generated by Xcode? but I got no answer so I decided to slightly change my question:

What happens when I choose this option here inside the images.xcassets section?

Does Xcode provide a full atlas manager and I just have to add single images inside this folder or should I put inside a whole atlas (created by my self with the .plist file attached)?