On the Scope definition page, you can specify what range of tasks is included in the scope of a given box (or a sub-box). Those are the tasks you will be able to work with in BigPicture (visualize them on a Gantt chart).
BigPicture gives you a lot of flexibility. You can include multiple projects, simultaneously include tasks from different external tools (such as Trello), or include only some tasks that meet your particular specifications. The flexibility of the setup ensures that a box can be configured to contain exactly the tasks you need to fit your work.
Keep in mind:
A box type contains the default settings applicable to multiple boxes (all boxes of a given type). The settings are adjusted in Administration > Box types.
Box configuration refers to the settings of an individual box and is dependent on the box type settings.
Further information can be found on the following pages: