Savedconfig

savedconfig is Article description::a USE flag that, in general, preserves the saved configuration files upon package updates. It allows for the updating of other files from a package update while, at the same time, guaranteeing no changes will be made to some specific files defined under Portage's directory.

Prerequisites
To understand how Portage handles configuration files, it is wise to the review the CONFIGURATION FILES section of emerge's man page:

By default, files under the directory are protected from modification or deletion unless Portage has been explicitly instructed otherwise.

Usage
Although it is possible to add  USE flag to the system's  file, it is likely to define it on a per-package basis. Setting  will restore the configuration files from the  file. Make sure the USE flags allow for appropriate dependencies.

Example
Enable the  USE flag on the  package:

Emerge the package; not that after package installation a message similar to the following will be displayed:

On the next emerge of the package, Portage will use the saved configuration file located at as the  file for compiling kernel. This file may be modified as necessary, or, for example, can be replaced by another entirely.

External resources
https://packages.gentoo.org/useflags/savedconfig - An online view of packages containing the  USE flag.