How to understand "Replaces" field in output of "apt-cache depends"?

Question

When I run apt-cache depends tar under my Debian 8.3 installation, then it says that it will replace cpio:

# apt-cache depends tar
tar
  PreDepends: libacl1
  PreDepends: libc6
  PreDepends: libselinux1
  Suggests: bzip2
  Suggests: ncompress
  Suggests: xz-utils
  Suggests: tar-scripts
  Conflicts: cpio
  Breaks: dpkg-dev
  Replaces: cpio
# 

However, I have both cpio and tar packages installed:

$ COLUMNS=80 dpkg -l cpio tar
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  cpio           2.11+dfsg-5  amd64        GNU cpio -- a program to manage a
ii  tar            1.28-2.1     amd64        GNU version of the tar archiving 

(edited by cas: the version Martin has installed is 2.11+dfsg-4.1+deb8u1 rather than the 2.11+dfsg-5 I have from debian sid, but it makes no difference. Faheem's answer is still correct for the same reason)

How to understand the Replaces: field in output of apt-cache depends?

Answer 1

Your question is missing some information.

apt-cache show tar 

Package: tar
Version: 1.28-2.1
[snip]
Replaces: cpio (<< 2.4.2-39)
[snip]
Conflicts: cpio (<= 2.4.2-38)

Package: tar
Source: tar (1.27.1-2)
Version: 1.27.1-2+b1
[snip]
Replaces: cpio (<< 2.4.2-39)
[snip]
Conflicts: cpio (<= 2.4.2-38)

Notice that it says it replaces cpio for version << 2.4.2-39. Similarly for conflicts. Now, I also have cpio installed.

apt-cache policy cpio

cpio:
  Installed: 2.11+dfsg-4.1+deb8u1
  Candidate: 2.11+dfsg-4.1+deb8u1
  Version table:
     2.11+dfsg-5 0
         50 tor+http://httpredir.debian.org/debian/ unstable/main amd64 Packages
 *** 2.11+dfsg-4.1+deb8u1 0
        500 tor+http://security.debian.org/ jessie/updates/main amd64 Packages
        100 /var/lib/dpkg/status
     2.11+dfsg-4.1 0
        500 tor+http://httpredir.debian.org/debian/ jessie/main amd64 Packages

But the versions are much larger than 2.4.x. This is some ancient issue that could probably be dropped from the current tar. Feel free to file a bug.

Answer 2

"Replaces" are packages that will have one or more files replaced by the package you are searching on. For even more detail, see the Debian policy guide: https://www.debian.org/doc/debian-policy/ch-relationships.html#s-replaces