Just starting with Mega and bit confused about the documentation.
The Arduino board communicates with the shield using the SPI bus. This is on digital pins 11, 12, and 13 on the Uno and pins 50, 51, and 52 on the Mega. On both boards, pin 10 is used as SS. On the Mega, the hardware SS pin, 53, is not used to select the Ethernet controller chip, but it must be kept as an output or the SPI interface won't work.
Product page also states the same
Arduino communicates with both the W5500 and SD card using the SPI bus (through the ICSP header). This is on digital pins 10, 11, 12, and 13 on the Uno and pins 50, 51, and 52 on the Mega. On both boards, pin 10 is used to select the W5500 and pin 4 for the SD card. These pins cannot be used for general I/O. On the Mega, the hardware SS pin, 53, is not used to select either the W5500 or the SD card, but it must be kept as an output or the SPI interface won't work.
The shield is compatible with UNO and Mega, so placed over and extends the same pins (0 - SCL & A5 - IOREF), it can't be placed over 50, 51, 52 on Mega. So how can Mega use these pins to communicate with the shield. Also, if I use the shield, can I not use them (50 - 53) for digital IO.