Job numbers can be recorded for sales orders, purchase orders and works orders.  Any text can be entered, but if it starts with a S, P or W then Herschel will assume its a sales, purchase or works order number.

Job numbers are particularly useful when you're making a works order for a particular sales order.  For example, if works order W00000012 is being produced to satisfy sales order S000000002, this can be recorded using the Make module's works order tool:

When the kit list for this works order is printed, Herschel will automatically find the sales order and insert the sales order details onto the kit list: