Database.com Metadata API Developer's Guide - Salesforce.com

Metadata TypesCustomFieldReadWriteSee Also:CustomFieldMetadataPicklist (Including Dependent Picklist)WeblinkCustomFieldRepresents the metadata associated with a custom field. Use this metadata type to create, update, or delete custom fielddefinitions. It extends the Metadata metadata type and inherits its fullName field. You can also use this metadata type towork with customizations of standard picklist fields, such as the Industry field for accounts.You must specify the full name whenever you create or update a custom field. For example, a custom field on a custom object:MyCustomObject__c.MyCustomField__cAnother example, a custom field on a standard object:Account.MyAcctCustomField__cDeclarative Metadata File Suffix and Directory LocationCustom fields are defined as part of the custom object or standard object definition. See CustomObject for more information.Note: Retrieving a component of this metadata type in a project makes the component appear in any Profile andPermissionSet components that are retrieved in the same package.Retrieving Custom Fields on Custom or Standard ObjectsWhen you retrieve a custom or standard object, you return everything associated with the object. However, you can also retrieveonly the custom fields for an object by explicitly naming the object and fields in package.xml. The following definition inpackage.xml will create the files objects/MyCustomObject__c.object and objects/Account.object__c.object,each containing one custom field definition.MyCustomObject__c.MyCustomField__cAccount.MyCustomAccountField__cCustomFieldVersionCustom fields are available in API version 10.0 and later.FieldsUnless otherwise noted, all fields are createable, filterable, and nillable.87