x y is by default interpreted as Coproduct[x, y].
x is by default interpreted as Coproduct[x].
Coproduct is used as an abstract dual to the operation of multiplication, most often in infix form.
See Section 3.10.4.
See also: \[Product] , \[Wedge] , \[Vee] , \[CircleTimes] , \[SquareUnion] .