Slitherine

The manual (pp. 53-54) states that there are six naval unit types. I need someone to explain how these relate to the units that exist individually in the game for purchase. For instance, if you purchase a submarine, does that unit represent 4 submarines or do you need to purchase 4 subs to create a submarine group? I am playing my first game as Axis and my starting fleets don't contain the exact ships described in the manual. If you have fleets that don't contain all the ships listed for a particular group, what does that mean? If you build fleets that correspond to the manual, do they perform better than other fleets? I am confused. Thanks in advance.

In the purchase screen you are buying whole groups at once.