ONET.XML defines the project and list defaults, as well as the base types for fields. It contains a definition of list templates available for creating new lists, the default items on the top link bar of a project, the document templates available, and the required base fields for all list types (defined in the BaseTypes section). During provisioning, or when a new list is created, the definitions in this file combine with those in a given list's SCHEMA.XML file in order to create the list's schema in the database. ONET.XML is located in the Program Files\Common Files\Microsoft Shared\web server extensions\50\Templates\1033\xml directory.
This file can be modified to change the list defaults, add more list templates, and add more default fields for a given list base type.
The most frequent changes you will probably need to make involve four top-level elements in ONET.XML, as follows:
TopMenuItems — Allows customization of the top link bar.
ListTemplates — For making custom lists available on the Create page in a Web site based on SharePoint Team Services from Microsoft.
DocumentTemplates — Allows you to specify document types for the New Document Library page in SharePoint Team Services.
BaseTypes — For customizing the base types of all your lists. Possible settings for base types are the following:
0 — Basic List
1 — Document Library
2 — unused
3 — Discussion List
4 — Survey
In the ListTemplates section, the SecurityBits attribute specifies the default security settings for lists, which can be set to allow full editing, read-only, or schema design roles for the members of a team Web site.