As a real application we are going to connect a generic 4-20 mA sensor to the 4-20 Sensor Board and power the sensor with the Board's 12 V output. This sensor is a type 2 transmitter. Type 2 transmitters are energized by the current loop, where the supply voltage is included in the receptor. The transmitter voltage is floating and the ground is in the receptor, so it is necessary to connect the corresponding jumper.
In the case of Plug & Sense!, the sensor can be connected to channel 1 (which is type 2 and type 3) using the terminal box probe connected to socket A. The two sensor wires are connected on positions 1 (channel +) and 2 (channel -).
After connecting the sensor, you only have to upload one of the example codes included in the Waspmote IDE, for instance the example number one: https://development.libelium.com/4-20-ma-01-current-loop-basic/
In the serial monitor you should see that the sensor is transmitting 4 mA. The 4 mA "bottom of span" signal allows the receiver to detect a broken wire or failed instrument. Remember that in normal operation, a 4-20 mA sensor delivers between 4 and 20 mA. Any current above or below means malfunction.
The constant-current feature of a current loop cancels out voltage drop errors due to long wiring runs (of course this would also be true if you selected different current values for zero and span [e.g., 5-30 mA]).