That means Amavisd will insert `X-Spam-Flag` and other `X-Spam-*` headers when email score >= 2.0. If you want to let Amavisd always insert these headers, you can set it to a low score, for example:
$sa_tag_level_deflt = -999;
Amavisd's main config file is different on different Linux/BSD distributions:
* Red Hat, CentOS, OpenBSD: `/etc/amavisd/amavisd.conf`
* Debian, Ubuntu: `/etc/amavis/conf.d/50-user` (and other config files under `/etc/amavs/conf.d/`)