It is a monotonically increasing number: * https://docs.freebsd.org/en/books/porters-handbook/versions/ https://docs.freebsd.org/en/books/porters-handbook/vers

It is a monotonically increasing number: * https://docs.freebsd.org/en/books/porters-handbook/versions/ https://docs.freebsd.org/en/books/porters-handbook/versions/ Mostly for large kernel- and userland changes that can alter ABIs, APIs, data structures, etc.