Extensible Field Types
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As you store your business information in Windows SharePoint Services 3.0, you may encounter instances where your important business data does not conform to the field types included in Windows SharePoint Services, or situations where you want to customize those general field types. Windows SharePoint Services gives you the ability to create custom field types, based on the robust set of base field types that are already included. These custom fields can include custom data validation, field rendering, and field property rendering and processing.
To create a custom field type, you must create the following:
A class that represents the custom field type. This class must inherit from one of the base field types in Windows SharePoint Services. This class can contain custom data validation for your field type.
A field type definition file. This is an XML file that contains the information that Windows SharePoint Services needs to correctly instantiate and run the field type. Optionally, the field type definition file can include information that defines how Windows SharePoint Services should render the field type in display and header mode, as well as how to render the field type properties on the field authoring page.
You can also render the field type properties by authoring an .ascx page to act as a field editor user control.