When typing a long integer number, using underscore characters between successive digits can improve readability. For instance, for the number 1000000000, it becomes 1_000_000_000 with underscore characters. This is easy to read. This is also the format defined in the Go spec https://go.dev/ref/spec#Integer_literals

For readability, an underscore character _ may appear after a base prefix or between successive digits; such underscores do not change the literal's value.

Kong cannot parse an int string like 1_000. It would return an error

expected a valid 64 bit int but got "10_000_000"