Visible milky way band is not a view of just one spiral arm. It is a view of the entire thickness of the galactic disc as viewable from our location within the disc. This band tells us that we are living in a disc shaped galaxy and it also gives hint about our location within the disc.
Suppose you are looking at milky way band in the night sky and start moving towards opposite direction while keep on looking at the band. When you will reach about 2000 light years away from the visible band — from that location Sun and surrounding stars will no more be visible. They would have become a part of the visible galactic band!