[Validation] Check If customer Ref No have already been used

The customer has a business rule that when entering customer reference numbers on sales orders it should not be possible to enter a Reference number that the selected customer has already used. 

• Check for already used Customer Ref Sample Package.bcp2 KB
• Check for already used Customer Ref.pdf600 KB
• 02 - Validación de número de referencia de clientes repetido.pdf500 KB