The SAM D21 Xplained Pro evaluation kit is an ideal choice for evaluating and prototyping with SAM D21 ARM Cortex-M0+ based microcontrollers, which is notably a powerful and high performance ARM Cortex.
The kit allows the developers to work with wide range of connectivity features like KSZ8081 Ethernet PHY via RMII, HS USB, SD card.The kit does not come with its extension board; instead it has to be purchased individually.
Note : SAM D21 Xplained Pro supports many Xplained Pro extension kits, including I/O1 Xplained Pro, OLED1 Xplained Pro, QT1 Xplained Pro, and PROTO1 Xplained Pro.Features:

SAMD21J18A microcontroller

One mechanical reset button
One mechanical user pushbutton – Wake-up, bootloader entry or general purpose
One yellow user LED
32.768kHz crystal
USB interface, device and reduced host mode
8 Mbit Serial Flash
3 Xplained Pro extension headers

Auto-ID for board identification in Atmel Studio
One yellow status LED
One green board power
LED Symbolic debug of complex data types including scope information
Programming and debugging
Data Gateway Interface: SPI, I2C, 4 GPIOs
Virtual COM port (CDC)

USB powered
Supported with application examples in Atmel Software Framework

Relevant resources:
Atmel SAM D21 Xplained Pro User GuideXplained Pro Hardware Development Kit (HDK) User GuideSAM D21 Xplained Pro Design Documentation
FAQs:
Q: What is the ATSAMD21-XPRO Evaluation Kit?
A: The ATSAMD21-XPRO is a development board from Microchip designed for evaluating and prototyping applications based on the SAM D21 family of ARM Cortex-M0+ microcontrollers.
Q: Which microcontroller is included on the board?
A: The board features the ATSAMD21J18A microcontroller, a low-power 32-bit MCU with various peripherals and capabilities.
Q: What are the key features of the ATSAMD21-XPRO Evaluation Kit?
A: It includes an on-board debugger, supports multiple Xplained Pro extension boards for adding functionality, and offers easy access to the microcontroller’s features through the Atmel Studio IDE.
Q:Is the ATSAMD21-XPRO Evaluation Kit compatible with Arduino?
A: While the board isn’t directly Arduino-compatible, the SAM D21 microcontrollers are supported by the Arduino IDE through third-party cores, allowing you to use the Arduino development environment if desired.
Q: What applications can I develop using the ATSAMD21-XPRO Evaluation Kit?
A: The kit is versatile and can be used for a wide range of applications, including IoT devices, wearable electronics, industrial automation, and other projects that require a low-power, feature-rich microcontroller.