When you create a Restriction for a User Role, you create a package with the Capabilities associated with that Role. If a Customer purchases access to a Restriction that protects a Role, they do not acquire the Role itself. Instead, they receive the Capabilities provided by that Role. See also: Roles and Capabilities in the WordPress Codex.
Note: There are certain WordPress Systemic Roles with special internal permissions that cannot be associated with a Restriction. These include:
shop_manager, and any other Administrator-like Roles detected by WooCommerce Restrictions in your installation of WordPress.
This is where you create new Restrictions. There is a meta box here called Protected Role Capabilities. Click the field to see the Roles available for packaging by WooCommerce Restrictions. Select the Role you want to sell from the drop-down list. When you have finished creating Restrictions, click the Create Restriction button at the top-right of the page.