When your customer requires you to provide a specific reference on your invoice, like a PO number, you can use the field Invoice Reference for this.
Now you can already provide this invoice reference when you are creating your instalments in the instalment wizard.

Here's how you do this:

1. Click the Add button on the instalments tab in your sales contract.

2. The system will open following screen and you can setup you first installment.

3. By clicking the Add button, you can add multiple installments if your contract requires this.

4. You can fill in your Invoice Reference in the fields provided.

5. Click save to create your installments.