What is smi_gpio used for?

On page 15 of the schematics, I noticed all the external interface into the rp2040. There is the usb, and serial for data transfer and for interaction with the rp2040, along with 2 reset pins. There is also a signal listed SMI_GPIO. What is this signal what is it used for ? If the user can use it, how is it setup up for input or output from the cpu side ?