Is the FrameMaker subscription run online? Does this mean I can use a Mac to access it?
No, the software runs on your device, requiring Windows.
Having said that, I've run FrameMaker exclusively on my Mac under vmware Fusion for years.
re: Is the FrameMaker subscription run online?
FM is not a cloud app. I have no particular guesses about whether it might ever become one. As I recall, Adobe at one time had an on-line demo mode for FM that was a pretty good mimic of a cloud version, if somewhat slow (as would have been expected for the time).
re: Does this mean I can use a Mac to access it?
As inferred by TCT, you would need to run the Windows version of FM under Windows 7 or 10 in a virtual machine on the Mac (x86 Mac). There are any number of people doing this, so it's not that much of a hack.
A native Mac version of FM is another future possibility, I suppose (it's asked for often enough), but running the current Windows version in a VM is the near-term solution.
I normally run VMware Fusion in full screen mode so all you can see is Windows & FrameMaker when I'm on that specific desktop, but I resized the Fusion window slightly so that you can see that FrameMaker is running on my Mac. Other popular options are Parallels and Bootcamp—all three apps allow you to install Windows on a Mac, allowing you to use FrameMaker and other Windows-only applications. I've been doing this for many years—I use multiple Mac desktops (aka Spaces) so I can just slide over to Windows when I need it—it's the best of both worlds.