RAK8211 blink program

Cellular Boards RAK8212
1

I have the RAK8211-NB, and blink works fine below:

int led_pin = 17;

void setup() {
pinMode(led_pin, OUTPUT);
}
void loop() {
digitalWrite(led_pin, HIGH);
delay(200);
digitalWrite(led_pin, LOW);
delay(200);
}

My question is, why should I use 17 instead of 29?