The APCA Number (sometimes called a "Direct Entry User ID") is an ID number that uniquely identifies your bank. It's often used when creating/managing electronic payments and in Easy Payslip when you create an ABA file.
For ANZ, NAB and Westpac, it will be six zeroes (000000).
For CBA it is 301500.
For any other institution, you'll need to contact them (or check in your online portal) to find the "APCA Number", "APCA ID" or the "Direct Entry User ID".
Learn more about Paying your Employees' Wages with an ABA File.