This is called “lexical ordering” and is basically a way to control the order of how all of the files are flattened together into the final configuration that your service is started with. Think in terms of 10 starts before 20, etc.
You can always view the contents and ordering with `systemctl cat foo.service`, which is a great way to make sure your etcd configuration turned out how you wanted it to.
All link files are collectively sorted and
processed in lexical order, regardless of the directories in which they
live. However, files with identical filenames replace each other. Files
in /etc have the highest priority, files in /run take precedence over
files with the same name in /lib. This can be used to override a
system-supplied link file with a local file if needed. As a special
case, an empty file (file size 0) or symlink with the same name
pointing to /dev/null disables the configuration file entirely (it is
"masked”).
- Rob