Via the menu item "Manage contacts" in the order management, you can store additional persons for your hosting package, to whom you would like to grant certain authorizations.
For example, you can define additional invoice recipients who will have access to invoices in addition to you and will receive them by e-mail, or you can give an administrator access to the customer menu (Plesk) without having to give him your access data.
As a reseller, you also create the additional domain owners that you need for the administration of your customer domains here.
If you want to give a contact the permissions for invoice and reminder notifications, you can find an example configuration in the "Invoice recipient permissions" image.
If you as a contract holder do not want to receive notifications about created invoices, you can disable this in your settings as shown in the image "Disable invoice sending for account holders".