where's customer registration definition in admin

Zen Cart has a login feature. But where is the registration button that accesses the registration that the customer ultimately logs in to?

Not sure what you're asking. A stock Zencart installation has a 'Login' link at the top of the header. If yours doesn't, we'll need to see the site.

How do customers register? They have to be registered before it's possible for them to log in. That's the first part of the question. The second part is: Where in the admin panel is that registration process defined/controlled?

They register by clicking the 'Login' link at the top of the header. You can (as I do) change that to Login / Register.

The fields in the Create account page are mostly controlled in Admin - Configuration - Customer Details and Admin - Configuration - Minimum Values.

Thanks ever so much stevesh. Your reply was so swift that I'm wondering if there's a way to configure the Zen Cart forums so that you are notified -i.e., "you've got mail", etc.- when someone replies to your forum posts?

Looks like Forum Actions- General Settings - Default Thread Subscription Mode