ARPU is your Average Revenue Per User. It is more granular than your ARPA (Average Revenue per Account). Because here, instead of calculating the average revenue generated by an entire account, you can calculate the revenue generated per user within this account.
To do the math, you will first need to calculate the average number of users per account: Total number of account / Total number of users.
So you can identify your
Average Revenue Per User = Total Revenue / Average Number of Users
This metric, just like the ARPA, will help you forecast and learn how many users you need to have in each account you will get on board to reach your revenue target.