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Questions relating to designing PCBs
By Piquan
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).