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Whether you're designing a blog template, or building a shopping cart, it helps to define user scenarios in order to engineer the process of conversion, or at least guide that process. That conversion may be simply to sign up for a newsletter, or subscribe to a web feed (yes, that's a conversion!) or it may be to purchase a combine harvestor, it doesn't make a lot of difference in general, the point is, that you should be aware of how the process should, could and does work in order to maximize a sites effectiveness.