I have a general question, please forgive me if the question is so trivial...
My question is: I have noted that all the Arduino modules have a couple of holes having apporximately a diameter of 2.5 mm (see AT24C02 modulo I2C interfaccia IIC EEPROM modulo di archiviazione smart car blue board|Wireless Module| - AliExpress just to have an example...)-
My question is: what's the meaning of these holes? How can be used while designing a custom shield?
Thank you very much and happy new year to everybody!