PIN Properties, Electrical type.

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Ground can be at a higher or lower voltage then the output ( a positive or negative voltage supply for example). From this perspective ground pins should be bi-directional.

GND electric type pin

Schematic of the DFPlayer mini pins

These are the pins Electrical types

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Power pins should be connected to power source, so when Kicad is doing the ERC, electrical rules check it can help us.

In conclusion, GND pin is electrical type is set to: POWER INPUT