Gets a value that indicates whether Microsoft SharePoint Foundation field type (as specified in the Type property) can be the parent of a custom derived field type.
Assembly: Microsoft.SharePoint (in Microsoft.SharePoint.dll)
An attempt is made to assign a value to a field type that is built into SharePoint. The true/false value of such fields must remain at its factory setting.
An attempt is made to assign false when the SharePoint field is an external data column. (For more information about external data columns, see Where Can You Show External Data?.)
This property is not about whether the SPField class, or any class derived from it, is sealed.