How do customer accounts and order history work?
If the customer is buying a membership or subscription they will have to create an account during checkout.
If the customer is buying any other type of product then you can decide if you want to require them to create a customer account.
The customer accounts feature for your store (and the ability to log in on your checkout page) can be configured in a few different ways. If the customer is purchasing is membership or subscription product, then the ability to log in is always shown on the checkout page. However, if the product is a digital product or a physical product with no membership or subscription associated with it, then whether or not the login link shows on the checkout page is determined by your Accounts setting here in the Cart66 management console: https://manage.cart66.com/accounts