a concret example can be explain here with your dhcpcd trouble (
https://forum.obarun.org/viewtopic.php?id=843).
Actually in the frontend file of dhpcd the command line is:
dhcpcd ${cmd_args} -f ${conf_dir}/${conf_file}
at the execution time 66 will replace the variable by the given one at /etc/66/conf/dhcpcd, meaning
cmd_args=!-B
cmd_args_stop=!-x
conf_dir=!/etc
conf_file=!dhcpcd.conf
So the command line become
dhcpcd -B -f /etc/dhcpcd.conf
i guess this part is clear for you. Just i explain here for other people.
So, now you want to specify the interface to use. To do so , you need to pass the name of the interface into the dhcpcd command line.
Well do
# 66-env -r 'cmd_args=!-B eth0' dhcpcd
now you /etc/66/conf/dhcpcd will contain:
cmd_args=!-B eth0
cmd_args_stop=!-x
conf_dir=!/etc
conf_file=!dhcpcd.conf
to appy this change at the daemon run
# 66-start -r dhcpcd
now the daemon use as command line
dhcpcd -B eth0 -f /etc/dhcpcd.conf
So you have modified the command line of the daemon without the need to change the entire frontend file.
In case of update of the daemon from upstream you don't loose your change except if you ask it to 66.
a command like
# 66-enable -f -S dhcpcd
will not overwrite the /etc/66/conf/dhcpcd file.
So you use the new script from the upstream but with your environment variable set previously
A command like
# 66-enable -C -f -S dhcpcd
will overwrite the /etc/66/conf/dhcpcd file and so you loose your change made previously