In this mode, the MX device does not provide any address translation and operates as a passthrough device between the Internet and the LAN ports (sometimes referred to as a Layer 2 bridge).
Placing an MX appliance in Passthrough mode at the perimeter of your network with a publicly routable IP address is not recommended and can present security risks. As a best practice, Passthrough mode MX appliances should always be deployed behind an edge firewall.
Thanks @AjitKumar. Yes I've read all this. The reason I ask this question is I'm doing some apps using the API and I was looking for a way to determine what mode the FW as in, since there are no API that return that.