TIdMessageEncoderClass = class of TIdMessageEncoder;

TIdMessageEncoderClass is a class type that identifies the class to which a TIdMessageEncoder descendant belongs. TIdMessageEncoderClass is the class type used to create new instances of a message encoder for processing items in the TIdMessage.MessageParts collection. 

For example, a TIdMessageEncoderUUE instance will have an EncoderClass property that contains a TIdEncoderUUE class type that is registerd in the Indy application. Subsequent instances of TIdMessageEncoderUUE can be created using the registered class type, reducing the resource requirements of the application.