Hi,
In reviewing the code from File > examples > BluetoothSerial >SerialtoSerialBT, i observe that the book example has additional line  of codes:
Line-9: // define UES_PIN // Uncomment this to use PIN during Pairing. The pin is specified on the line below
Line-10: const char *pin = “1234”; // change this to more secured PIN 
Line-12: String device_name = “ESP32-BT-Slave”;
My interest on the use of password. 
I could not connect to the device when line-9 was uncommented. I thought the Serial Bluetooth Terminal on an  mobile android would need to input the pin to enter into pairing with the server.
I could find the menu on the android app to enter the pin.
How di I use this feature?
Please also review line-28 of the current example :
//Serial,printf(“The device with the name……..   ……. SerialBT.getMacString()); // Use this after MAC method is implemented.
What is this feature and how to use it?