apt line - Archive: pool

 deb http://snapshot.debian.net/archive pool package ...
 deb-src http://snapshot.debian.net/archive pool package ...

Useful when you get some specific version of the package but you don't know exact date when the version is installed in archive.

Since this apt line labeled as "NotAutomatic: yes" in Release file,
any version in this apt line will not be automatically upgraded.
You should specify target archive like this:

 apt-get -t pool install <package>
 apt-get -t pool install <package>=<version>
 apt-get -t pool source <package>