Billing Codes let you create various options that can override the chart of accounts settings you define when you first connect Syngency to Quickbooks.
For instance, you may have account 261 in Quickbooks assigned to "Agency Expense (Model)" (expenses charged to the model by the agency):
But in the case of it being a cash advance to the model, rather than an expense, you may want to use an alternative Quickbooks account when entering this in Syngency.
First, you would add it under Settings:
And then each time you enter a Fee, Expense, or Chargeback, you have the option of selecting a Billing Code to assign it to the respective Quickbooks account: