I dont really see PLC caring about whether a contact is physically NO or NC or if it is pump to full or pump to empty or spring to position or anything like that In the description section it is said In ladder diagram the contact symbols represent the state of bits in processor memory which corresponds to the state of physical inputs to the system If a discrete input is energized the memory bit is a  and a normally open contact controlled by that bit will pass a logic true signal on to the next element of the ladder So if the input contacts are physically closed the corresponding input address is high and have a  bit in the internal memory which happens when the tank is empty as per the description So that should have been the condition for latching the valve right