pt-query-digest $event->{bytes} >= 1_048_576 why "_" in there?

in the doc
Query is 1 MB or larger
$event->{bytes} >= 1_048_576
why not $event->{bytes} >= 1048576? what is ‘_’ mean?

Is 1M the size of sql text? what is the calculation method?
select length(“select * from xx where id=1”) ?


In Percona Server, the slow log contains a line with the number of bytes sent like:

# Bytes_sent: 41

That can be used in filters as


Regarding the underscore in numbers: