3-column files
A signed Amount column usually uses positive values for deposits and negative values for payments, but confirm the bank's convention.
Transaction basics
Credit and debit columns need context.
The same sign can mean different things for a bank account and a credit card. Use the account type, not the column name alone.
4-column files
Credit and Debit are separate positive columns. A valid row normally has a value on only one side.
Credit cards
A payment to a credit card may appear as a credit or debit depending on the source system. Check the target account and a small sample of known transactions.
Never silently flip
A formatter should show the direction it inferred and let you correct it. It should not reverse every amount because a header looks familiar.