The percent sign is something I once saw in Drupal 6 and 7. I didn't see it in views in Drupal 8; I don't know what was the reason to use this as it was in codes I've seen in a glimpse (It was a few years ago, I don't recall where exactly inside views but most probably inside filter criteria regarding a filtering via URL).
My question can actually be comprised of the following two questions.
What is % in View's arguments?
Is it something unique to Linux/PCRE/Drupal-itself?