The process formation is the array of processes that are used to do the app’s regular business (such as handling web requests) as it runs.
Twelve-factor strongly favors languages which provide a REPL shell out of the box, and which make it easy to run one-off scripts.
In a local deploy, developers invoke one-off admin processes by a direct shell command inside the app’s checkout directory.
In a production deploy, developers can use ssh or other remote command execution mechanism provided by that deploy’s execution environment to run such a process. Cheers!!