There are three options for Invoice setup: No Invoice, New Invoice or Attach Job to Existing Invoice
As seen in the image below, when you click on the Invoice dropdown, you have three choices:
No Invoice: this will dispatch the job with no invoice attached.
Add New Invoice: this will open the invoice page with all contact details and payment terms automatically populated, ready to simply add the invoice line items for the job, or
Attach Job to Existing Invoice: this will present all existing invoices related to the client selected, enabling you to assign the job to an existing invoice. This is particularly useful for businesses that invoice before doing the job.