Thanks. I’m trying, but I’m still not sure.
Sorry. I’m paraphrasing with examples.
Manjaro branches differ only in the frequency with which packages in directories are maintained by a bot (in our case arm-stable, arm-testing, arm-unstable). Where Pacman reaches is set with # pacman-mirrors --api --set-branch <my_choice>.
The dev variant of Manjaro/PlaMo takes KDE Plasma Mobile applications packages from directory kde-unstable, the regular one from community. I don’t know what exactly was meant by “image is based on unstable branch”. Dev branch of arm-profiles?
Images are created for the purposes of flashing. Dev by sudo buildarmimg -d pinephone -e plasma-mobile-dev -v 20210226 -b unstable command, regular by sudo buildarmimg -d pinephone -e plasma-mobile -v 20210226 -b stable -s plasma-mobile -o (nice article 
).
Correct?