Usefulness of adding APT::Default-Release

[David Wright <deblis@lionunicorn.co.uk>]

I'm trying to ascertain what APT::Default-Release can do for me,
and what it constrains. In the output that follows, why does
APT::Default-Release prevent firefox from being upgraded?

I comment out the APT::Default-Release line and repeat after the
##########. The necessary packages are in apt-cacher-ng's cache
all the time.

# cat /etc/apt/apt.conf
# Fetch updates through apt-cacher-ng.
Acquire::http::Proxy "http://192.168.1.17:3142/";;
APT::Default-Release "stretch";
# 

# apt-cache policy firefox-esr
firefox-esr:
  Installed: 60.6.1esr-1~deb9u1
  Candidate: 60.6.1esr-1~deb9u1
  Version table:
     60.6.2esr-1~deb9u1 500
        500 http://ftp.us.debian.org/debian stretch-updates/main amd64 Packages
 *** 60.6.1esr-1~deb9u1 990
        990 http://ftp.us.debian.org/debian stretch/main amd64 Packages
        990 http://security.debian.org/debian-security stretch/updates/main amd64 Packages
        100 /var/lib/dpkg/status
# 

# apt-get dist-upgrade
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Calculating upgrade... Done
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
# 

#############################################################

# cat /etc/apt/apt.conf
# Fetch updates through apt-cacher-ng.
Acquire::http::Proxy "http://192.168.1.17:3142/";;
##APT::Default-Release "stretch";
# 

# apt-cache policy firefox-esr
firefox-esr:
  Installed: 60.6.1esr-1~deb9u1
  Candidate: 60.6.2esr-1~deb9u1
  Version table:
     60.6.2esr-1~deb9u1 500
        500 http://ftp.us.debian.org/debian stretch-updates/main amd64 Packages
 *** 60.6.1esr-1~deb9u1 500
        500 http://ftp.us.debian.org/debian stretch/main amd64 Packages
        500 http://security.debian.org/debian-security stretch/updates/main amd64 Packages
        100 /var/lib/dpkg/status
# 

# apt-get dist-upgrade
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Calculating upgrade... Done
The following packages will be upgraded:
  firefox-esr firefox-esr-l10n-en-gb
2 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Need to get 43.0 MB of archives.
After this operation, 3,072 B of additional disk space will be used.
Do you want to continue? [Y/n] ^C
# 

# cd /var/cache/apt-cacher-ng/debrep/pool/main/f/firefox-esr/
# ls -l *6.2*b ; cd
-rw-r--r-- 1 apt-cacher-ng apt-cacher-ng 42433100 May  6 12:00 firefox-esr_60.6.2esr-1~deb9u1_amd64.deb
-rw-r--r-- 1 apt-cacher-ng apt-cacher-ng   520268 May  6 12:00 firefox-esr-l10n-en-gb_60.6.2esr-1~deb9u1_all.deb
# 

Cheers,
David.