An enumeration denoting the source of the event that causes the trigger to fire.
To call an enum, you call its parent class, name, and property. For example, 
ScriptApp.TriggerSource.SPREADSHEETS.
Properties
| Property | Type | Description | 
|---|---|---|
| SPREADSHEETS | Enum | Google Sheets causes the trigger to fire. | 
| CLOCK | Enum | A time-driven event causes the trigger to fire. | 
| FORMS | Enum | Google Forms causes the trigger to fire. | 
| DOCUMENTS | Enum | Google Docs causes the trigger to fire. | 
| CALENDAR | Enum | Google Calendar causes the trigger to fire. |