apt and do-release-upgrade question - what's sources.list.distUpgrade for?

[Chris Green]

I have been running do-release-upgrade on a couple of systems to move
from 19.10 to 20.04, all went very smoothly, thank you Ubuntu
maintainers.

However I see in /etc/apt there are three versions of sources.list
after the upgrade:-

    chris$ ls -l sources*
    -rw-rw-r-- 1 root root 3315 Jul 20 10:22 sources.list
    -rw-rw-r-- 1 root root 3304 Jul 20 10:22 sources.list.distUpgrade
    -rw-rw-r-- 1 root root 3304 Nov 24  2019 sources.list.save

    sources.list.d:
    total 24
    -rw-r--r-- 1 root root 174 Jul 20 10:22 dhor-ubuntu-myway-bionic.list
    -rw-r--r-- 1 root root 174 Jul 20 10:22 dhor-ubuntu-myway-bionic.list.distUpgrade
    -rw-r--r-- 1 root root  81 Jul 20 10:22 syncthing.list
    -rw-r--r-- 1 root root  48 Jul 20 10:22 syncthing.list.distUpgrade
    -rw-r--r-- 1 root root 139 Jul 20 10:22 vincent-vandevyvre-ubuntu-vvv-bionic.list
    -rw-r--r-- 1 root root 139 Jul 20 10:22 vincent-vandevyvre-ubuntu-vvv-bionic.list.distUpgrade

Ignoring the stuff in the sources.list.d (though that follows the same
pattern), what is sources.list.distUpgrade?  I can see that sources.list.save
is just my old/previous sources.list renamed which is handy to see if
anything needs re-enabling after the upgrade.  But sources.list.distUpgrade
just seems like 'noise'.  I've always removed them in the past, after
the upgrade has completed, but it feels as if it would be better if
the upgrade cleaned up after itself.

-- 
Chris Green