If you are not creating your sales invoices within Reviso but outside the application and just need to record the sale against the customer and within your accounts you can use the option Manual sales invoices. it maybe necessary to record these invoices with different number series.
For example, you may have different business sectors, sales and service. These can be differentiated by the number series making them easily identifiable.
You can make your selection from the entry screen once the new number series can been created within Configuration > number series > new number series.