Note the lip aroumd the top of the tank used for mounting. On the Polyethylene tank shown, the mounting is the wide formed ledge seen on the side or end of the tank.
Note the mounting ledge on the side or end of the tank versus the lip on the top of the ABS tank. The fitting on the tank will be the same type of plastic as the tank although it may be a different color. The drain pipe will be clamped or threaded into this fitting.