In Spark you can found two main types of items: Hot Keys and Actions.
HotKeys:
A Hot Key represents a keystroke. You can create two kind of HotKey: Simple HotKey that simply executes an Action when you type the associated keystroke, and custom Hotkey that execute differents Action, depending the frontmost Application.
Actions:
An Action defines what append when a HotKey is executed. A HotKey must contain at least one Action, the default Action.
You can found more info about actions by reading "Actions Categories"
Application:
You can also find other kinds of items, like Application.
When you create a Custom HotKey, you can associate one action per Application.
You can also group Applications into List, and then associate an Action with an Application List,
i.e. if one of the Application into this list is the frontmost Application when you type keystroke,
Spark have to execute the action you choose.
Spark defines a special Application: "System (default)". This Application represents the default behavior of your HotKey.