apt.conf evaluation order

[Josef Wolf]

Hello,

Can anybody point me to information about evaluation order of the apt.conf
files? The apt.conf manpage does not specify whether /etc/apt/apt.conf is
read before or after /etc/apt/apt.conf.d/* and in which order the files
in apt.conf.d are processed. Are they processed via run-parts? How should
backup files be named to be ignored?. Do late settings override previous
settings or is a setting immutable if it is once set?

For example, /etc/cron.daily/apt states that the admin should set his
preferences in /etc/apt/apt.conf.d/02periodic to override the defaults.
But how can this work if /etc/apt/apt.conf.d/10periodic (contained in
update-notifier-common) sets its own values? From that I'd assume settings
are immutable?

Can anybody help to clarify?