TIdBaseStream = class(TIdStream);

TIdBaseStream is a TIdStream descendant that acts as an ancestor for classes that require a platform-specific stream implementation. TIdBaseStream implements overridden Read, Write, and Seek methods to isolate the differences in parameters used as arguments to these methods. 

TIdBaseStream declares virtual abstract methods that must be implemented in descendant classes to provide the functionality for the Read, Write, and Seek operations. 

TIdBaseStream is used as the ancestor for TIdMultiPartFormDataStream used in POST method requests in TIdHTTP.