Что такое amd64 в “dpkg-l” вывод для некоторых пакетов а не для других?

Следующее является выводом dpkg -l команда в моей системе Ubuntu,

Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name                            Version                                    Architecture Description
+++-===============================-==========================================-============-===============================================================================
ii  adduser                         3.113+nmu3ubuntu3                          all          add and remove users and groups
ii  apt                             1.0.1ubuntu2.19                            amd64        commandline package manager
ii  apt-utils                       1.0.1ubuntu2.19                            amd64        package management related utility programs
ii  base-files                      7.2ubuntu5.5                               amd64        Debian base system miscellaneous files
ii  base-passwd                     3.5.33                                     amd64        Debian base system master password and group files
ii  bash                            4.3-7ubuntu1.7                             amd64        GNU Bourne Again SHell
ii  bsdutils                        1:2.20.1-5.1ubuntu20.9                     amd64        Basic utilities from 4.4BSD-Lite
ii  busybox-initramfs               1:1.21.0-1ubuntu1                          amd64        Standalone shell setup for initramfs
ii  bzip2                           1.0.6-5                                    amd64        high-quality block-sorting file compressor - utilities
ii  console-setup                   1.70ubuntu8                                all          console font and keymap setup program
ii  coreutils                       8.21-1ubuntu5.4                            amd64        GNU core utilities
ii  cpio                            2.11+dfsg-1ubuntu1.2                       amd64        GNU cpio -- a program to manage archives of files
ii  cron                            3.0pl1-124ubuntu2                          amd64        process scheduling daemon
ii  dash                            0.5.7-4ubuntu1                             amd64        POSIX-compliant shell
ii  debconf                         1.5.51ubuntu2                              all          Debian configuration management system
ii  debconf-i18n                    1.5.51ubuntu2                              all          full internationalization support for debconf
ii  debianutils                     4.4                                        amd64        Miscellaneous utilities specific to Debian
ii  dh-python                       1.20140128-1ubuntu8.2                      all          Debian helper tools for packaging Python libraries and applications
ii  diffutils                       1:3.3-1                                    amd64        File comparison utilities
ii  dmsetup                         2:1.02.77-6ubuntu2                         amd64        Linux Kernel Device Mapper userspace library
ii  dpkg                            1.17.5ubuntu5.8                            amd64        Debian package management system
ii  e2fslibs:amd64                  1.42.9-3ubuntu1.3                          amd64        ext2/ext3/ext4 file system libraries
ii  e2fsprogs                       1.42.9-3ubuntu1.3                          amd64        ext2/ext3/ext4 file system utilities
ii  eject                           2.1.5+deb1+cvs20081104-13.1ubuntu0.14.04.1 amd64        ejects CDs and operates CD-Changers under Linux
ii  file                            1:5.14-2ubuntu3.4                          amd64        Determines file type using "magic" numbers
ii  findutils                       4.4.2-7                                    amd64        utilities for finding files--find, xargs
ii  gcc-4.8-base:amd64              4.8.4-2ubuntu1~14.04.4                     amd64        GCC, the GNU Compiler Collection (base package)
ii  gcc-4.9-base:amd64              4.9.3-0ubuntu4                             amd64        GCC, the GNU Compiler Collection (base package)

Из чего означает :amd64 в конце некоторых имен пакета т.е. e2fslibs:amd64 или gcc-4.8-base:amd64? Почему архитектура не указана для всех пакетов?