Default List Content Types
Applies to: SharePoint Foundation 2010
In Microsoft SharePoint Foundation 2010, every item has a content type assigned to it, either implicitly or explicitly. You explicitly set the content type of an item by selecting it for that item. However, even items that haven't explicitly been assigned a content type by a user must still adhere to the metadata requirements, or schema, of the list in which they are located. And so the list schema actually functions as the default content type of the list itself.
By design, the base content type hierarchy included in SharePoint Foundation maps to the various types of lists you can create. When you create a list, SharePoint Foundation copies a list content type, based on the appropriate base site content type, onto the list. Create an announcement list, and SharePoint Foundation copies the Announcement site content type locally to the list as the default list content type; create a document library, and SharePoint Foundation copies the Document content type locally to the list as the default list content type; and so on. So although the user may not be aware of a list having a default content type, it always does.
If you make changes to a list, such as adding or deleting columns, you are also in effect making changes to the list content type included on the list by default.
For more information about the content types included with SharePoint Foundation by default, see Base Content Type Hierarchy.