On an Arduino Uno, pins 9, 10, and 11 are part of a specific group of pins with dual-purpose functionality:
These pins are critical for SPI communication, which is used to talk to SD card readers, RFID modules, and high-speed displays: Pin 10: SS (Slave Select) Pin 11: MOSI (Master Out Slave In) Pin No 9 10 11
Using certain libraries (like the Servo library) can disable PWM functionality on pins 9 and 10 because the library "takes over" the internal Timer 1. On an Arduino Uno, pins 9, 10, and