Mer is a 100% open source project, Sailfish OS is not. Mer is Mer and Mer is part of Sailfish. Read this http://releases.merproject.org/~cars...014_vfinal.pdf

It explains a bit about what Mer is in relation to Jolla and check this http://gitweb.merproject.org/gitweb?o=age to see what changes have been made to Mer and by who.