Hi Woodsee,
Thank you for providing the detailed background with your inquiry.
As far as entering the job description into the subject line of the emails sent, this is unfortunately not possible in terms of the way you are describing how you want it (automation).
There is no list of what can be [inserted] as the only piece of data that gets pulled in the subject line of the emails being sent is [InvoiceNo]
I would suggest putting the job description in the box labeled Message on Invoice located underneath the line items whilst creating the invoice.
Kristian.