If you buy a perpetual retail license for Office 2013, it will be locked to the computer you first install it on, forever. Buy a new PC and you won't be allowed to install your existing copy of Office on it, even if you wipe the disk of the old PC. You'll have to splurge for a new one.
This is a change in policy from Office 2010. Office 2010 permitted a single transition from one PC to a new one. It's not, however, an entirely new policy: OEM pre-installed versions of Office (and Windows) are similarly tied to their (OEM) hardware and can't migrate. Adam Turner at The Age first pressed Microsoft for clarification over what its "single PC" constraint actually meant, and noted the newly aligned OEM and retail licenses.
It's difficult to see the wisdom in this change. It's not a big change, but it's not a nice one, either.
Retail sales make up a minority of the Office business. Microsoft doesn't habitually report the exact level of retail sales, but we can perhaps make estimates based on the information the company does provide.
The Microsoft Business Division (MBD), the reporting group within the company that includes Office, Exchange, SharePoint, Dynamics, and Lync, reported last quarter that 60 percent of its revenue is from multi-year subscriptions—Software Assurance plans. The remaining 40 percent is what Microsoft calls "transactional;" one-off purchases, encompassing both OEM preinstalls and boxed copies bought online or in bricks-and-mortar stores.