postlogin script

I'm using the postlogin service, following the examples in the wiki. But I can't find any documentation on the behavior (what's allowed/not allowed) of the script-login binary. So, some questions:

Question 1:

The examples show the following at the end of the post-login.sh script: exec "$@"

My understanding is that this would exec each of the command line arguments to the post-login.sh script. But, there are no arguments sent to the post-login.sh script in the examples. So what is this line supposed to do?

Question 2:

One of the examples shows exporting some environmental variables, followed by the above exec line:

export MAIL=maildir:/tmp/test

export USERDB_KEYS="$USERDB_KEYS mail"

exec "$@"

Now, I'm really confused. Can someone explain step-by-step why this does anything at all?

Question 3:

I'd like to be able to pass some information to the post-login.sh script, such as the service (%s), as a positional parameter.

For example: executable = script-login /path/post-login.sh %Ls

Or even more explicitly: executable = script-login /path/post-login.sh imap

But it appears that the script-login binary is expecting only script names to be passed to it so that it can handle more than one script. Is there a way to pass arguments to the different scripts?

Thanks,

Michael