To summarise, the development model of FreeBSD can be seen as the following tree:
The tree of the FreeBSD development with ongoing development efforts and continuous integration.
The tree symbolises the release versions with major versions spawning new main branches and minor versions being versions of the main branch. The top branch is the -CURRENT branch where all new development is integrated, and the -STABLE branch is the branch directly below it.
Clouds of development efforts hang over the project where developers use the development models they see fit. The product of their work is then integrated into -CURRENT where it undergoes parallel debugging and is finally merged from -CURRENT into -STABLE. Security fixes are merged from -STABLE to the security branches.