The promo codes that give you extra free domain registrations are like the one free one you get with every hosting plan. You get however many domain registration credits every year.
So in your example, if you used a promo code that gave you one extra domain registration, you'd get two credits per year (counting the first one everyone gets) and would be able to have two domains registered forever (as long as you maintained your hosting plan).
Note that you don't have to keep the same domain every year. You can use the "credits" to either renew your existing domain or register a new one and let the old one lapse. The one thing you have to be a little careful of is that you get your credits annually in your last month of each year so you can't "switch for free" in the middle of the year.
Is the hosting any good? Check out the various posts here. You'll find lots of people here who are happy and some who are sad. A lot depends on what your expectations are and whether you are lucky in being assigned to a trouble-free server - that is, a server with nice neighbors.
What are you expecting to do with your hosting account?
What are [color=#CC0000]50DISK50[/color], [color=#CC0000]3DOM50[/color], and [color=#CC0000]1IP1DOM50[/color]?
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