While SharePoint allows you to create many different types of site columns, you will find yourself at times creating custom site column types. For example, you may want to create a new site column type that displays a drop-down list that gets data from the database or some other source. To create a custom site column type you need to create the following:
• A new field type class. This is a class that must inherit from
Microsoft.SharePoint.SPField or one of its inherited classes.
• A new field type definition. This is defined in an XML file that has a naming convention such as fldTypes_*.xml and is deployed to <14 hive>\Template\XML. The field definition file tells SharePoint information related to the field type such ...