Thanks. Ah, okay, I see, so in general, it would seem preferable to use -t, which means that I've been doing it wrong. :-(

Just to qualify this statement (that I made), I guess that there are situations where you would want to install package X from backports while minimizing other changes to your existing setup, in which case

apt-get install X/beowulf-backports

would be the better strategy. However, the risk with this strategy (as I found out), particularly if X has a number of dependencies, is that one of the dependencies (which is not installed from backports) is too old or isn't fully compatible with X from backports.

This is why the safest strategy would be to install X from backports using

apt-get -t beowulf-backports install X

From the man page:

-t, --target-release, --default-release
This option controls the default input to the policy engine; it creates a default pin at priority 990 using the specified release string.

Using /beowulf-backports doesn't change the default release so APT can only attempt to draw the dependencies from beowulf.

apt install roundcube/beowulf-backports roundcube-core/beowulf-backports roundcube-mysql/beowulf-backports

Thanks. Ah, okay, I see, so in general, it would seem preferable to use -t, which means that I've been doing it wrong. :-(

At the same time, the emacs metapackage did pull in the intended emacs version (27.1) from backports, which is why I thought that

apt-get install emacs/beowulf-backports

did what I thought that it did. In other words, in the case of emacs,

apt-get install emacs/beowulf-backports

seemed to me to do what

apt-get -t beowulf-backports install emacs

would have done (but it probably didn't do exactly the same thing after all).

Using /beowulf-backports doesn't change the default release so APT can only attempt to draw the dependencies from beowulf.

apt install roundcube/beowulf-backports roundcube-core/beowulf-backports roundcube-mysql/beowulf-backports

That's just a metapackage. Try

apt install -s emacs-nox/beowulf-backports

For example, I can install emacs from backports unproblematically using the following command:

apt-get install emacs/beowulf-backports

Suppose that I follow the same strategy to install roundcube from backports:

apt-get install roundcube/beowulf-backports

This results in an error message saying that roundcube can't be installed because one of its dependencies, roundcube-core, can't be installed, which puzzles me.

However, if I try to install roundcube from backports using the command

apt-get -t beowulf-backports install roundcube

then it works!

Does anyone know why there is this difference?