This magic number constant should be replaced by a #define or const int

https://github.com/arduino/ArduinoCore-avr/blob/9f8d27f09f3bbd1da1374b5549a82bda55d45d44/cores/arduino/USBCore.cpp#L404

Was looking at why I'm not able to emulate a keyboard with 8-byte bMaxPacketSize0. Took me a while to hunt this one down, was looking for 64 all over but found 0x3F.
Suggest:
#define bMaxPacketSize0_64 64
Then use (bMaxPacketSize0_64-1) for clarity.

I know it's just my opinion but this USB device stack could use more clarity and documentation. It would make it much easier to expand.

Open cores/arduino/USBCore.cpp at the linked line and inspect how the 0x3F value is used in the USB device stack. Replace the unexplained literal with a named constant as suggested, then verify that the relevant expression remains equivalent and that the file builds successfully.

Written by the indexing model from the issue text.