Are you trying to add the job numbers for your customers to differentiate between them? If so, we can utilize the Description field as a workaround, geoff.
Please be aware that adding a specific job number field directly to a line item in an invoice is not possible.
In the invoice creation page, input the necessary job numbers in the description field:
You can also utilize the subtotal feature to group each job for easier tracking of how much they owe separately: