Here is an update and summary on the Core/Strategic definition Fraction discussion as of late January 2016, based on mailing list discussions, comments, and Wes's and others' input from the Annual Planning perspective:
There are three different uses identified for this data, all slightly different. We need to be careful (and have been so far, I think) that data collected for one purpose is not used in a different context.
The most urgent and primary definition of Core and Strategic for Product and Engineering is for Annual Planning. This is driven by adopting the FDC format for WMF's planning. The definitions for this purpose are the ones on this page. Departments will be splitting their work into Core and Strategic and submitting their Core plan before their Strategic plan, though both plans will follow the same format. All Core and Strategic plans will be reviewed before anything is finalized (so it shouldn't happen that Core plans eat up the whole budget allocation before any Strategic plans get a shot); I don't have any more detail on budgeting and prioritizing.
While the definitions seem clear in isolation, lots of people have had questions about specific cases, and it's not hard to think up tricky ones. (Is adding Traditional Chinese character support to VE Core or Strategic? What about fixing a bug in it two years after it's launched?) Since we already tried both meetings and mailing list discussion to clarify this, without final results, I suggest that the most efficient path forward is for Departments to continue splitting up work according to this definition as they create their plans, using their best interpretation, and to post especially tricky cases on the wiki for feedback and discussion.
I'll update on the two other use cases in a seperate thread; they are distinct and less urgent.