I worked for a manager that would literally cringe when someone would say “couch” He would retort “we don’t carry couches, only sofas!” Pretentious huh? I worked there for so many years that the word “couch” is inevitably out of my vocabulary.
Couch is a generic term used in the US, Australia & New Zealand to refer to sofa. However the original word is a derivative from the French word “Couche” meaning “bed”. The traditional couch was a seat with at least one raised end and maybe a half raised back like what we would refer to today as a chaise lounge or day bed.
A couch is therefore a piece that should be used to “lay-down” as opposed to a sofa or settee that is used for “sitting up”
So is a settee the same as a sofa? Not exactly! A settee is a long seat with a hard back; preferably a decorative hard back (as shown below).
And of course the sofas which we normally use in our living and family room seats two or more, fully upholstered in either textile or leather. They may have arms or not and most commonly have multiple cushions.
I have used them all for spaces I have designed, after all they all have a special place.