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When you look at the diagrams for the drive mode at page 10 of pin's datasheet you will see that the output is driven only to one direction.
The "Open Drain drives low" will be connected to GND when the pin is written a 0 to. A pullup resistor will be needed to maintain the high state of the pin. This configuration allows for more than one devices actively driving the pin low as for instance I2C uses for clock stretching.
"Open Drain drives high" will work accordingly when a 1 is written to the pin.