What does mean pi_ prefix?

Question

I've found that some plugins have the prefix "pi_", for example:

/usr/share/nvim/runtime/doc/pi_spec.txt
/usr/share/nvim/runtime/doc/pi_tar.txt
/usr/share/nvim/runtime/doc/pi_msgpack.txt
/usr/share/nvim/runtime/doc/pi_health.txt
/usr/share/nvim/runtime/doc/pi_tutor.txt
/usr/share/nvim/runtime/doc/pi_paren.txt
/usr/share/nvim/runtime/doc/pi_zip.txt
/usr/share/nvim/runtime/doc/pi_gzip.txt
/usr/share/nvim/runtime/doc/pi_netrw.txt

Those files are of neovim 0.4.4

Just for curiosity, what does mean the "pi_" prefix?

Answer 1

I'm pretty sure it stands for "plugin."