A .pacnew file may be created during a package upgrade (pacman -Syu, pacman -Su or pacman -U) to avoid overwriting a file which already exists and was previously modified by the user. When this happens a message like the following will appear in the output of pacman:
warning: /etc/pam.d/usermod installed as /etc/pam.d/usermod.pacnew
A .pacsave file may be created during a package removal (pacman -R), or by a package upgrade (the package must be removed first). When the pacman database has record that a certain file owned by the package should be backed up it will create a .pacsave file. When this happens pacman outputs a message like the following:warning: /etc/pam.d/usermod saved as /etc/pam.d/usermod.pacsave
These files require manual intervention from the user and it is good practice to handle them right after every package upgrade or removal. If left unhandled, improper configurations can result in improper function of the software, or the software being unable to run altogether.
to check out what needs manual adaptation