Parameters SERVICEINPUTCLASS and SERVICEOUTPUTCLASS - what are they?

Several Ensemble adapters have SERVICEINPUTCLASS and/or SERVICEOUTPUTCLASS parameters defined.

For example:

Class EnsLib.HTTP.InboundAdapter Extends EnsLib.TCP.InboundAdapter
{

Parameter SERVICEINPUTCLASS = "%Stream.Object";

Parameter SERVICEOUTPUTCLASS = "%Stream.Object";

...
}

What are they for?