Variable

Underscore

• Typically, the underscore is used in front of member variables in a class, to distinguish them (for the reader) from local variables. The presence or absence of the _ (or __) makes no difference to the compiler.
• In C++, an underscore usually indicates a private member variable.
• Names beginning with an underscore or a double underscore are RESERVED for the C++ implementers. Names with an underscore are reserved for the library to work.

REF

• https://forum.arduino.cc/index.php?topic=48820.0
• https://stackoverflow.com/questions/3136594/naming-convention-underscore-in-c-and-c-sharp-variables