Why am I seeing this message about 'esm-apps' on just one system?

[Chris Green]

I have several systems running [x]ubuntu and I update/upgrade them
manually every week or so.

On just one of these systems I'm seeing the following when I run 'apt
upgrade' :-

    root at esprimo# apt upgrade
    Reading package lists... Done
    Building dependency tree... Done
    Reading state information... Done
    Calculating upgrade... Done
    Get more security updates through Ubuntu Pro with 'esm-apps' enabled:
      libimage-magick-perl imagemagick libavdevice58 imagemagick-doc ffmpeg
      libopenexr25 imagemagick-6-doc libpostproc55 libmagickcore-6.q16-6-extra
      libavcodec58 libimage-magick-q16-perl libmagickwand-6.q16-6 libavutil56
      imagemagick-6.q16 libswscale5 libmagickcore-6.q16-6 libswresample3
      imagemagick-6-common libavformat58 libavfilter7
    Learn more about Ubuntu Pro at https://ubuntu.com/pro
    The following packages have been kept back:
      apparmor grub-efi-amd64-bin grub-efi-amd64-signed libapparmor1 shim-signed
    0 to upgrade, 0 to newly install, 0 to remove and 5 not to upgrade.
    root at esprimo# 


Have I inadvertently enabled/selected something on that one system
that makes this happen? ... or what?

-- 
Chris Green