LittleOne,
Remember that current signals are "loop" signals--that is when all wires are connected, they form one big loop that must be continuous.
Keeping that in mind, visualize your DC power coming in on + Pin 1 (Brown), going through the internal resistance of the transmitter, where the miliampere current gets modified by the pressure, then coming back out on Pin 2 (white), then going to the Analog module input terminal, through the internal resistance of the analog module (where the miliampere current is "read" by the PLC), then out on the "Com" terminal of the Analog Module, then finally back to the - side of your DC power supply.
The DC power (17 to 33 Volts) can be either internally supplied by the PLC rack, or supplied by an external DC power supply. In your case the - volts are also fed into Pin 3 (Blue), probably for internal use by the transmitter. Some transmitters need this internal power, others are "loop" powered (get their power from the signal loop), and some are duel use, meaning that they can be loop-powered or separately powered from a DC power supply.