Activating Office 2011 is a known issue for Fusion Drive Macs.
[Some users are reporting on Apple Discussions that 10.9.2 has fixed the issue]
Activation is tied to the drive. According to Microsoft the solid state part of the Fusion Drive generates a new disk id every time the Mac is started. So Mac Office thinks it a new system at each startup and will require activation.
This solution was suggested by Microsoft Support. Please . . .
Continue Reading: Fusion drive solution to activate Office for Mac 2011
Microsoft introduced Activation with Office 2011. Since Office 2011 was first released, Office365 subscription service was added. The retail copy of Office for Mac and the Office365 Subscription service use different activation processes. The number of installs also differs. The retail version will depend on what version you purchased (one or two computer install, or family pack that installs on 3 computers). more info. The Office365 Subscription service will install on 5 computers more info.
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Continue Reading: Microsoft Office Activation FAQs