SPBuiltInContentTypeId.System field

Contains the content identifier (ID) for the System content type.

Namespace:  Microsoft.SharePoint
Assembly:  Microsoft.SharePoint (in Microsoft.SharePoint.dll)

public static readonly SPContentTypeId System

All content types ultimately inherit from the System content type, and the System content type ID is the root of all content type IDs.

For more information about the lineage of content types, see Base Content Type Hierarchy.