Average monthly withholding amount (AMWA) is used to determine which type of payroll remitter you are: regular remitter, quarterly remitter, or accelerated remitter.
It is calculated by adding up all the payroll deductions you have to remit to the Canada Revenue Agency (CRA) for your payroll accounts two calendar years ago. And then you divide the total by the number of months (maximum 12) that you had to make payments in that year.
For example, if you made two monthly remittances totaling $120,000 in 2010, your AMWA for 2012 would be $60,000 ($120,000 divided by 2), and you would be a Threshold 2 employer.
The CRA will calculate AMWA for you and they will notify you if your remittance type changes based on their calculations usually in December.