Ubuntu 20.04 packages with an "{u}" in their name

Question

After an upgrade to Ubuntu 20.04, some packages have "{u}" in their name. I have looked for the answer, but it looks like either google is promptly ignoring the curly braces during search or this is does not happen much. Can someone help me with links to documentation? What does the {u} signify in these package names?

user@server:~# sudo aptitude update && sudo aptitude upgrade && sudo apt-get autoremove
Hit http://mirrors.linode.com/ubuntu focal InRelease
Hit http://mirrors.linode.com/ubuntu focal-updates InRelease
Hit http://mirrors.linode.com/ubuntu focal-backports InRelease
Hit http://security.ubuntu.com/ubuntu focal-security InRelease
Hit http://ppa.launchpad.net/ondrej/php/ubuntu focal InRelease

The following packages will be REMOVED:
  ack{u} libargon2-0{u} libfile-next-perl{u} libicu65{u}
  python-pkg-resources{u}
0 packages upgraded, 0 newly installed, 5 to remove and 0 not upgraded.
Need to get 0 B of archives. After unpacking 34.4 MB will be freed.
Do you want to continue? [Y/n/?]

Answer 1

The curly-brace suffixes only serve to clarify that those package actions (in this case, removal of {u}nused packages) were not specifically what you asked for in your command, but automatic, derived decisions by aptitude.

In this case, you asked to remove any package no longer depended upon by other manually installed packages, and aptitude identified those 5 packages.

Depending on constraints defined in the installed and available package (versions), any installation, upgrade or removal can lead to additional actions performed. Those actions are not necessarily limited to one choice or an obvious choice - hence the command output makes very clear which packets were selected because you mentioned them, and which were chosen automatically (in case this choice does not match your expectations).

At the Do you want to continue? prompt, the third option (type ? instead of y or n) brings up the explanation, so if anything looks odd you can always get the relevant documentation, even if future versions add more suffixes than currently explained in this relevant Q&A on our sister site Unix & Linux: aptitude: Meaning of curly brackets, i.e. package1{b} package2{a} package3{u}