How does the AI tool know how to code expenses to the correct accounts/chart of accounts?

The AI has a general understanding of accounting practices. You can direct it to use GAAP standards explicitly (telling it to look up GAAP standards online as its reference), or you can feed it your own trial balance / chart of accounts so it learns your account numbering for sales, labor, cost of goods sold, etc. You can also have it review existing QuickBooks files/ledgers to learn your firm's typical categorization style and then apply that framework going forward. You can give it ongoing feedback ('I don't like how you did that, do it this way instead') to keep refining its behavior.