It is provided for convenience to include simple but usually sufficient components in an AR scene. You can build a different AR scene using other smaller prefabs provided inside EasyAR.
An EasyAR prefab should be used with ImageTarget Prefabs. They together construct the whole AR scene.
You can read APIs for other prefabs and behaviours for more detailed descriptions.
In the EasyAR 1.3 and new versions, EasyAR prefab is a primitive contains a building script. You can set Key in the inspector. Read this for more details.
The previous EasyAR prefab has been renamed to EasyAR_Startup. And that contains a ImageTracker.