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Questions relating to designing PCBs
#200655
Yes, it's for the 0.1" pitch 2x3 ICSP header footprint, just like the one on an Arduino.

Pogo pins don't strictly need holes, although a hand-soldered pogo jig like this is easier to get lined up if you have them. Just stick with your usual via size; anywhere between 0.6-1.0mm works well (just testing with my pogo adapter and a template I have with various hole sizes drilled in it).