The term current sinking and current sourcing, from a logic gate, means the ability of how much a device (or pin) can sink current through the load or source current to the load. Below shows such types, when an output pin is high or 5v, this provides a voltage to the connected components, which allows current to flow, this is sourcing. And when a pin is low or 0v, this provides a current path through it as the components are connected to the positive power supply, this is sinking. Looking at the configurations below they all light the led but two circuits are sourcing and the other two are sinking. When looking at data sheets, if a device states that it can source 16mA , however 25mA is required, look for the sinking current and it may be able to sink this amount, and if so then a sink configuration could be used. Other methods could also be used like buffers, transistor drivers to assist in sourcing current.