In 'the old days' there were distinct differences between different sizes of trucks: light trucks had a small engine and were intended for light loads on good roads. Medium duty trucks were the link between light and heavy-duty trucks, and primarily intended for distribution transport only, while heavy-duty trucks were intended for a wide variety of purposes, from construction-site tasks to long-distance transport.
Mixing a number of great features
Today, the improved efficiency of engines even of smaller size together with computer-aided-design mean that it is possible to design 'smaller' trucks which can perform heavy-duty tasks, at least as long as no trailer is attached to the truck.
At the time the F6S was introduced it was mostly the heavier family member 'F7' which was the focus of the attention. But the F6S was in fact a very interesting truck, where Volvo had created a mix of a medium-duty chassis, rugged design and a very efficient turbocharged engine with a performance well in excess of its formal engine capacity.
A "small" heavy-duty truck
The F6S was a close relative of the F6 truck presented in 1975. The chassis was fairly similar to the design of the heaviest F6 version (F613/F614) but strengthened even more. Even though the F6S was considered as a powerful medium-duty truck, its formal GVW of 16,000 kg (earlier 15,500 kg) meant that it was strictly speaking a heavy-duty truck, but intended for a smaller range of transport applications than the more powerful Volvo trucks with larger engines.