POS only products should only be enabled if you require some products to be hidden from either the online or physical store. Please see the POS Only Products doc for more information.
A new order will initialise with the default POS customer. Common options are:
Guest: POS orders are not associated with any user id
Cashier: POS orders are associated with the logged in user's account
Walk-in Customer: POS orders are associated with a custom user account (note: this account will need to be created)
WooCommerce POS Pro users are able to create user accounts on-the-fly during checkout. An email address is required to create a new account, this is a requirement of WordPress. Creating an account for new customers is recommended as it means you can track their order history and potentially contact your customers with promotional offers in the future.