How is the default 'visits_evolution' metric calculated?

I’ve searched online and have been combing through the matomo codebase, but I can’t seem to find the past date or past period that is being used to calculate the ‘visits_evolution’ metric from the default api call module=API&method=MultiSites.getAll&period=day&date=today.

It says here how it’s calculated

((currentValue - pastValue) / pastValue) * 100

…which makes sense…but they don’t say where they are getting the value for pastValue. Is it the nb_visits for the prior day? week? month?

Can someone help?