The SIGNAL() is a macro to define a function to register to interrupt vectors.
The SIGNAL() is defined in hardware/tools/avr/avr/include/avr/interrupt.h as below.
The ‘signal’ in the _attribute_ means that it is an interupt handler. The ‘used’ tells the compiler to write code to a file unless it looks unused. The ‘externally_visible’ invalidates the effect of the ‘-fwhole_program’, which is a gcc compiler option.
Arduino 1.8.13/avr-gcc 7.3.0-atmel3.6.1-arduino
June 19, 2020