This shows two extreme cases of tax shifting possibilities.

In the top graph, demand is completely inelastic. You can change price all you want and there will be no effect on the quantity sold. In this case, it is possible to pass all of the tax on to the buyer. Tax revenues are equal to T x Q.

Since Q is fixed, the Government knows that the amount of tax revenue is determined by the tax rate, T.

In the bottom graph, demand is completely elastic. In this case, even a small change in price completely eliminates effective demand. Therefore, producers will pay all of the tax. They can not pass any of it on to consumers. Tax revenues are again equal to T x Q, but Q depends on the level of taxes and the location of the supply curve. In this case, the excise tax is more effective in reducing consumption than in raising tax revenues.