TIdDecoderClass = class of TIdDecoder;

TIdDecoderClass is a TIdDecoder class reference that determines the object type created when a new instance of the decoder is created. TIdDecoderClass is passed as an argument in the DecodeString function, and used to create the internal class instance that performs the decoding operation. 

Pass TIdEncoderClass as an argument to the EncodeString function to perform encoding for a string value.