ARPU is the general form of the revenue-per-user family. The period and the denominator matter: monthly ARPU on active users answers a different question from ARPU on an install cohort over 30 days.
In games, ARPU on a cohort basis is the building block of LTV: sum a cohort’s revenue over time and divide by the users who installed. In subscription apps, ARPU is often quoted monthly across all active users, including non-payers.
Always state the window and the denominator when you report it, or two teams will compare numbers that do not measure the same thing.