The bottom line: Microsoft went in the third direction Computerworld listed as a possibility in February. "The most expedient solution would be to tie Office Large to Office 365," Computerworld speculated then. "In that case, it would probably count Office Large as one of the five allowed PC or Mac installs for each user, not as one of the five smartphone or five tablet installs."
But Computerworld got that last bit wrong. The Office for Windows 10 licensing agreement stated, "Word, Excel, PowerPoint and OneNote are licensed for your personal, non-commercial use. Under this agreement we grant you the right to install and run one instance on up to each of ten (10) of your Windows 10 devices (the licensed devices) that are affiliated with the Microsoft account associated with your Windows Store account."
So Office for Windows 10 doesn't count against either the PC or the mobile device install allowances, but against the 10 total for each user. If a user has already maxed out the five-installs allotment for personal computers, they could still install Office for Windows 10 on, say, a sixth PC running the new OS.
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