A generic term for sausage that is in casing — e.g., what people normally expect sausages to be. Sausages that aren’t stuffed into casings include Lorne Sausage, Pan Sausage, sausagemeat, etc.
It is also used, in America, to denote sausages that aren’t in casing — rolls of sausagemeat in plastic wrap that you make slices from and fry up.
When someone uses the term “tube sausage”, you have to figure out what they mean as you go.