dpk-query: why are certain packages being shown as “un … (no description available)”?

Solution: Some other package depends on this package, but only optionally resp. as an alternative; of the alternatives … one is indeed installed, but not this one, but still this is gets listed.

This is how the package first showed up:

$ dpkg-query --list foo...
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/...
|/ Err?=(none)/...
||/ Name Version Architecture Description
+++-======================-================-================-...
un foo___________________ <none> <none> (no description available)

I have that package “foo…”  being displayed as Unknown (Desired) and Not (Status).

I am rather keen to know, why it gets listed at all.

I cannot find the right dpkg* command to tell me, which package depends on foo… .

This is sort of a “raw command” to help me:

$ fgrep --before 10 foo... /var/lib/dpkg/status

Update 2018-05-30:

This command line (almost) does, what I expect:

$ apt-rdepends --reverse foo... # the "r" in "rdepends" stands for "recursive"

The quirk is, that the dependency in fact is only optional resp. alternative (this is the reason, why the package is not installed), but this fact does not get displayed.

This command line does not, what I expect:

$ apt-cache rdepends foo... # the "r" in "rdepends" stands for "reverse"

• https://manpages.debian.org/unstable/apt-rdepends/apt-rdepends.1.en.html