This won't come as a huge shock to anyone, but Microsoft has confirmed, via a footnote in a promotional blog post, that editing (as opposed to viewing) Office documents on Windows 10 Mobile with Continuum will be tied to the requirement for a valid Office 365 subscription. As with the Windows licenses generally, it's all tied to screen size, so plugging your 5"-screened smartphone into a 25" 1080p monitor in order to be able to use Office on the phone in the same way as on a desktop understandably needs to trigger the same kind of licensing rule.

At the moment, editing Office documents is unrestricted for those playing with the Lumia 950/XL and the Display Dock (etc.), but as the whole system is in beta (effectively) then the requirement isn't being enforced until the end of March 2016.

In practice, the requirement for Office 365 is unlikely to affect many people unduly, since anyone who owns a 950/XL, a Display Dock or Display Adapter and who's also invested in the world of Office is likely to already have an Office 365 subscription - it's by far the chepaest way to 'buy' both Office and OneDrive cloud storage these days. 

From the Office blog:

Office Mobile + Continuum—Continuum is a new Windows 10 feature that allows you to use your phone like a PC. When you connect your phone to a larger screen, keyboard and mouse, your Office Mobile apps scale up so you can work on Office documents like you would on a PC*. While using Office on a larger screen, you can continue to use your phone and other mobile apps separately for multitasking.

Cortana and Office 365 Integration—Earlier this year, we added the ability for Cortana in Windows 10 to connect to Office 365 so she can help you accomplish more at work. By connecting with your Office 365 work account, Cortana can proactively help you prepare for upcoming meetings, pull together information about the people you are meeting with and their relevant documents. Now this integration is available on Windows 10 phones, so you can be more productive with less effort, even on the go.

*Editing in Word, Excel and PowerPoint with Continuum for phones will require an Office 365 subscription, but is available to try without Office 365 through March 31, 2016.

Comments welcome, of course, if you disagree with me and think this is unfair?

There's also a rather nice little promotional video around Office on Windows 10, which I hadn't seen before, embedded below for your interest: