This is an app settings to let you choose the calculation formula for your Sales/Purchase, you can choose one to setup for your EMERGE App account depends on your preference.
Round tax to 2 decimal place on line item
price x quantity x tax per line item then round up each line item tax individually
Round tax to 2 decimal place on document subtotal
price x quantity x tax per line item, does not round up each line item tax but all summed in total amount
Check out this sample excel file to understand the difference.