29.5.1 I/O Lines
Other than the internal GCLK sources, the clock provided on the REFI line is the external clock input source for GCLK_FREQM_REF/GCLK_FREQM_MSR, which can be used for measurement or reference clock sources. This requires the I/O pins to be configured using PPS configuration. See I/O Ports and Peripheral Pin Select (PPS) from Related Links.