Assignment for Attributes in external Scripts

Hi, I am writing an external script to be run based upon an authentication. When the script returns output I am unsure as to what the assignment operators do, for example when should I use += vs := vs = If I look at the exec-program-wait sample script I see: echo "Reply-Message += \"Hello, %u\"," echo "Reply-Message += \"PATH=$PATH\"," echo Framed-IP-Address = 255.255.255.255 Is this documented anywhere? thanks, niall