adduser: what is the difference between --disabled-password and--disabled-login

[Shaul Karl <shaulk@013.net>]

adduser(8) states that 

    With the --disabled-login option, the account will be created but
    will be disabled until a password is set. The --disabled-password
    option will not set a password, but login are still possible for
    example through SSH RSA keys.

I wonder what is the difference? Alternatively, how adduser accomplish 
that? The relevant source lines seem to be:

    } elsif ($arg eq "--disabled-password") {
    $ask_passwd = 0;
    $disabled_login = 0;
    } elsif ($arg eq "--disabled-login") {
    $ask_passwd = 0;
    $disabled_login = 1;
    }

    if ($ask_passwd) {
    &systemcall('/usr/bin/passwd', $new_name);
    } else {
    if(!$disabled_login) {
        &systemcall('/usr/sbin/usermod', '-p', '*', $new_name);
    }
    }

Perhaps what I really should have asked is about the contents of
/etc/{passwd,shadow}'s password field for disabled accounts.