To expand on Vladimir's comment:
Assuming 24 vdc I/O cards.
On a transistor output card Sourceing is PNP or Active High. This means that you connect the output to the + side of the load device and connect the other side to 0vdc. If the output Card is Sinking it is NPN or active low meaning that you would tie the + side of the device to 24vdc and the common would connect to the output card.
On input cards if the input card is a sourceing card you would use NPN sensors. The cards common connection would be to a +24vdc source and the device (sensor) would give a path to 0vdc. If the input card was sinking you would use PNP sensors. The sensor would be connected to the + 24vdc source and the output from the device would connect to the PLC
input card which would have a common connection to 0vdc.