The curtain veil has a very strong symbolism, particularly evoking the veil of the Holy of Holies in the Temple. The curtain marks the hidden and ineffable presence of God. The veil is used on a tabernacle or hanging pyx, and it is an abuse to reserve the Blessed Sacrament with only a lamp and no veil. The curtain is drawn inside the royal doors of a Byzantine iconostasis. I have also seen heavy black drapes on the doors of the choir screen in York Minster, but I think their purpose is more to do with keeping the cold out rather than God in!

See:
http://www.newliturgicalmovement.org/2008/01/bringing-verticality-and-presence-back.html

You will find metal curtain rails between the baldachino posts, rather like the rails you have above the windows of your house, even though the curtains are no longer in use and have totally disappeared.

The relationship between an English altar with riddel posts and the old Roman altar is obvious.

To answer your question specifically, the dossal curtain is simply one of the four curtains. The curtain between the altar and the church is eliminated, so only three of the four curtains remain. The dossal curtain is sometimes replaced by a reredos, which developed in size and complexity in the baroque era, featuring only two pillars instead of the original four.

It is important to notice these details so that we can see the essential unity of liturgical traditions.