The Trade Account section is there so you can input a regular customer or bill payer so as you can quickly select them as the ones to bill when adding a new job. For example, if you did work regularly for British Gas, you would add them as a Trade Account so when you book a job request from British Gas, you can select them as the bill payer, not the customer.
Bad Trade Accounts? If you have any Trade Accounts that turn bad (non payment etc), you can mark them as 'On Stop' from within Trade Accounts so then if another job is booked and gets assigned to them for billing, you'll get a big red warning saying they're on stop. This will stop any further debt being built up without realising.