[Question] What is the purpose of the number on the end of the directory names?

I haven't been able to discern from the code - what is the purpose of the number at the end of the recorded directories

e.g. recording_dir/suite-name_234234234234/test-name_3423423423423

Also, is there a particular reason why the test name is a directory called test-name containing recording.har rather than a file called test-name.har ?