Well, it is not “just” the version number. It’s a new development model.
Leap will take some (most) of the core base system packages (not the kernel though) from SLE (SUSE Linux Enterprise, the commercial distribution), and they will be maintained by SUSE.
The rest comes from Tumbleweed, like with 13.2 and earlier.
At least for the first release, many things that would have been taken from SLE12 had to be updated to the Tumbleweed version already (Xorg e.g.), because the packages in SLE12 were too old and broke things with newer other packages (e.g. GNOME not starting).
The release schedule will be aligned with SLE as well, i.e. whenever a new SLE Service Pack is released, there will be a new Leap release (again with some packages from SLE, most from the them current Tumbleweed).
Will Tumbleweed remain or will Leap take on both roles? I’d like to move to the fast track, but only if it more stable than Tumbleweed is today
Tumbleweed will remain as it is.
Leap is a stable release model, Tumbleweed is the rolling distribution.
One distribution cannot be both.
What will be the role of FACTORY?
Exactly the same as now. The place where the development for Tumbleweed happens.
OTOH, SLE is also based on Tumbleweed anyway, just an older version.
Leap is not much different to current stable releases, except that some core packages will come from SLE and will be maintained by SUSE.
Factory = fastest updates / lower stability
There is no way really to install/use Factory, it doesn’t exist as distribution.
I wonder just how “far behind” or “slower” is Leap going to be compared to traditional, ordinary release-cycle based distributions (e.g. Ubuntu, Fedora, Mint, etc.)?
I don’t think there will be much difference in the end.
Many packages that would have been taken from SLE had to be upgraded to the Tumbleweed version already anyway because of problems (the packages are too old for current desktops like the latest GNOME3 and Plasma5).
And Leap will be am “ordinary release-cycle based distribution” anyway. It’s just that the release cycle will be aligned to SLE’s Service Packs, and with every release it will be reconsidered which packages to take from SLE and which ones from Tumbleweed AIUI.