That’s a leftover from the origins of this board, which started out as an auxiliary controller for the L3X-Z hexapod robot. One requirement of our selected scenario at ELROB 2023 was to be able to detect radioactive sources. Hence I purchased a simple (yet expensive) radiation sensor which provides a counts/minute (CPM) signal that was captured by that pin =) Clearly the documentation needs to be cleaned up a bit @generationmake 
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For reference, here’s already a thread dedicated to the OpenCyphalPicoBase board: https://forum.opencyphal.org/t/opencyphalpicobase-a-node-with-raspberry-pi-pico-and-mcp2515 .