Contemporary descriptions of older ships, at least from the late 19th century onwards, often refer to the uppermost (continuous) deck as an awning deck, sometimes as a spar deck, or even as a shelter deck. I gather that this was one level above the main deck, a term which seems to have retained the same meaning to this day. It also appears that the volume thus enclosed was regarded as only partially contributing to the gross/net tonnage, and accordingly, that the quantity and/or type of cargo permitted in that space was restricted.
Perhaps these terms are no longer relevant, but I have been unable to find their definitions, or any explanation of the differences between these types. Nor is it clear why the space enclosed by such a deck should not count in full to the tonnage measurement.
Anyone in a position to clear away the fog? Thanks in advance.