Like, the other commenter said, it's YoY, and a subtle mistake to think you can compartmentalize it to monthly is YoY / 12. Often what can happen is a sudden spike, like 8% inflation, where every month for the next year would turn out as 8% even though the month-to-month is flat every month except for the one sudden 8% jump.

Monthly = YoY / 12 iff inflation is linear and steady. (it almost never is)