confused about GPIO vs IO

[wjquigs]

A development board manufacturer included the attached diagram of the ESP32 module. I'm confused as to the relationship between GPIO and IO. Why would GPIOs be mapped to completely arbitrary pins? If I want to connect a CAN transceiver, can I simply #define CAN_RX_PIN GPIO_NUM_26 and connect the module's RX to GPIO 26? If so what's the point of describing the IO pins at all?

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