Additional information on which pins can be used for which functions is available on the pin information page.
The Tracker SoM has shared A and D pins. In other words, pin A0 is the same physical pin as pin D0, and is also the SDA pin. The alternate naming is to simplify porting code from other device types.
|Pin||M8 Pin||Function||Function||Analog In||GPIO|
|A0 / D0||Wire SDA||✓||✓|
|A1 / D1||Wire SCL||✓||✓|
|A2 / D2||Serial1 CTS||✓||✓|
|A3 / D3||3||Serial1 RTS||✓||✓|
|A4 / D4||SPI MOSI||✓||✓|
|A5 / D5||SPI MISO||✓||✓|
|A6 / D6||SPI SCK||✓||✓|
|A7 / D7||SPI SS||WKP||✓||✓|
|TX / D8||5||Serial1 TX||Wire3 SCL||✓|
|RX / D9||4||Serial1 RX||Wire3 SDA||✓|
On the Tracker One and Tracker Carrier Board you must enable CAN_5V in order to use GPIO on M8 pins 3, 4, and 5 (A3, D8/TX/SCL, D9/RX/SDA). If CAN_5V is not powered, these pins are isolated from the MCU starting with version 1.1 of the Tracker One/Tracker Carrier Board (September 2020 and later). This is necessary to prevent an issue with shipping mode, see technical advisory note TAN002.