combine subscribeOn , what is the meaning of upstream?

https://developer.apple.com/documentation/combine/publishers says

"subscribe(on:) changes the execution context of upstream messages."

I did a little experiment.

Publishers.Just(1)

.map { _ in

print(Thread.isMainThread) // true

}

.subscribe(on: backgroundQueue)

.map { _ in

print(Thread.isMainThread) // false

}.sink {

print(Thread.isMainThread) // false

i expected false false false. but results was true false false

What does 'upstream' mean specifically?