Expense account is a Type; Category is some sort of additional cross-reference.

Example:

My total sales in UF to select is $5,000 but I see I incurred a $10 Wire Fee for a payment to me made electronically. In reality, my Bank Deposit was $4,990. That means I select the $5,000 of customer payments and I enter the Negative $10 as Bank Fee Expense to me. The reason I Lost that amount, is the Expense account I put it against.

Net deposit = $4,990 "Net" = The Math.