Questions about using any of the apt tools should use this tag. Advanced Packaging Tool, the package manager for Debian-based distributions. It is also the most used tool for installing packages in Ubuntu.
APT uses dpkg (Debian package management system) as a backend. APT downloads the package list, keeps track of all packages installed, solves dependencies and removes unnecessary packages. It also allows you to query the cache, upgrade packages, manage packages from multiple repositories, etc.
Any question about the APT itself or its tools should use this tag, this include:
apt-config: to manage APT configuration.
apt: command-line interface of apt, available from apt 1.0 (Ubuntu 14.04). Plans to replace several utilities.
apt-key: manage the different GPG keys for repositories
apt-cdrom: manage CD-ROM entries
apt-cache: query the apt cache
apt-mark: to mark specific packages as automatically or manually installed
apt-get: the default package handling utility
Questions about editing or configuring APT itself using the files in the directory
/etc/apt (e.g. pinning) and questions about other directories that are also related to APT, and the usage of the different APT APIs, calls and libraries, like python-apt should all be tagged with apt.
There are other front ends for most of the functions of the tools included in the apt package, like aptitude, wajig and dselect.