Pmtr can be found here: http://troydhanson.github.io/pmtr/. It is a minimal process supervisor typically used to run a collection of locally-developed processes from a single config file, while pmtr itself runs under systemd.
A user requested a snap to install pmtr. I have pushed this release: https://dashboard.snapcraft.io/snaps/pmtr/revisions/1/
Process supervisors like pmtr need unconfined access to the system. Their runtime configuration determines the processes they spawn. Of course, these spawned processes have open ended requirements of their own. For example, the user may be configure pmtr to run a network daemon and a GStreamer application. Classic confinement is needed for pmtr to launch these, and for the child processes to inherit the same privileges.