Your invoice number setting is the number that your invoice numbers will start being generated from within Commusoft. 'Manual' means you will type a number each time you create an invoice, and 'Sequential' means you set up a number from this page and the invoices will go from there.
In order to set this up, open your system settings and click into 'Invoice number' from the 'Invoicing' section.
You will see a page with tick boxes, click on the option you would like your invoice numbers to start from and then click 'Save' once you've made your selection.