The Invoice Number field is used to record the invoice number once you have created an Invoice for a Job Request.
So for example:
You have a chargeable Job Request for some plumbing work one of your employees/contractors has just completed. The cost for the job was $100.00. You create an Invoice using your accounting package, then you go back into myPSR and record the Invoice number that was raised for that Job Request.
If you don't record the invoice number and the Job Request was a chargeable request then the Not Invoiced tab in the Dashboard home page would display all the Job Requests that were chargeable and have not been invoiced.
This allows you to cross check and make sure that all Job Requests that were chargeable have in fact been charged to your customers.