Those who buy Surface Laptops get a free one-year Office 365 Personal subscription which will allow them to download and use the "Office in the Windows Store" apps.
Office 365 Personal, Office 365 Home, Office 365 for Education Plus, and Office 365 Education E5 users will all have access to the Office in the Windows Store apps during the preview period, enabling them to work with Windows 10 S.
Those with Office 365 business plans are advised to upgrade their Windows 10 S devices to Windows 10 Pro and install a business version of Office, as "Office in the Windows Store does not currently support Office 365 business plans or volume licenses," according to the support article.
"When the preview period is over, other Office editions also will work on Windows 10 S, including Office Home & Student 2016 and Office Home & Business 2016. In 2018, Office 365 plans for business customers will also be supported," the support article says.
On June 15, according to the Support article, "Word, Excel, PowerPoint, Outlook, Access and Publisher" will be part of "Office for the Windows Store." (Outlook, Access, and Publisher won't be available in preview for Office 365 Education plan users, however.)
As a reminder, the Office in the Windows Store suite is not identical to the full desktop Office suite. Those using Office in the Windows Store can install the 32-bit version only, not the 64-bit version. Users of the Office in the Windows Store release cannot use COM add-ins on Windows 10 S, but can on other Windows 10 versions. And the OneNote app that is installed will not be the OneNote 2016 version; it will be the Universal Windows Platform version.
Windows 10 S users currently have the option to use the UWP versions of the Office apps that are in the Windows 10 Store now. But Microsoft is repositioning these UWP apps as "Office Mobile apps" that are recommended for phones and tablets with screen sizes less than 10.1. Windows 10 S users on devices with screens larger than 10.1 inches can install these Mobile/UWP apps if they'd prefer.
Microsoft developed the Office for Windows Store suite using its Centennial Desktop Bridge, which enables developers to bring their Win32 apps to the Windows 10 Store. Apps developed using Centennial are purchasable, downloadable, and able to be updated via the Store.
Update (June 15): To clarify, Microsoft is making Office in the Windows Store apps available to Windows 10 S users only for now. "It (Office in the Windows Store) will be available for purchase in the Windows Store by the end of this year," a Microsoft spokesperson confirmed.