Schema.org extensions versus core

There has been a request to clarify when something should go into an
extension versus when something should go into the core. Here is a first
stab at clarifying that.

For something to be in the core, the following conditions must be satisfied:

1. There must be at least a 1000 sites that will use it. Preferably 10,000+
2. It must have at least 10m weekly users. Preferably 100m
3. The vocabulary must be relatively compact. Less than 20 terms.

Of course, these are not hard constraints. We also recognize that
vocabularies evolve and more usage than planned might happen. We expect
terms or entire vocabularies to move from the extensions to core and vice
versa.

This is a start of the discussion.

guha