As you may already know, you can install SharePoint Server 2010 (not Foundation) with a Standard licence, which enables some features.
Then, when you discover that you need more or want to enable the Enterprise Features (such as Form Server), you can go to the Central Administration and then enter the new product key to enable Enterprise features.
The following is really interesting.
If you want to enter a new product key to enable Enterprise feature, you go to the Central Administration, then click on the link called ‘Convert farm license type‘, available on the Central Administration home page.
And then…. you can’t enter any thing on the ‘Enter the product key’ field.
To be able to enter the new product key, you have to go to the Central Administration\Upgrade and Migration section and then click on ‘Enable Enterprise features‘.
As you have install SharePoint with Standard licence, you have the choice to enable Enterprise feature, which then display a field where you can enter the product key.