How is the Quantity in Stock of a product updated in Vtiger?

The stock is updated automatically using the default workflow UpdateInventoryProducts On Every Save. This workflow is active by default and adjusts stock levels whenever invoices or purchase orders are created, modified, or canceled.

The benefit of this workflow is that it reduces manual effort and avoids human errors, as every change is recorded automatically. You can also create other workflows to run on Products by going to Settings> Automation> Workflows.
Follow the steps below to create workflows:

1. Log in to the CRM.
2. Click the Main Menu.
3. Go to the Settings.
4. Select Workflows under the Automation tab.
5. Click Add Workflow.

To learn more about Workflows in Vtiger CRM, click here.
To learn more about Product Stock Update, click here.

Here are the scenarios under which Invoices impact Quantity in Stock: 

• When an invoice is created, the quantity in stock will decrease.
• When an invoice is canceled or deleted, the quantity in stock will increase.

Here are the scenarios under which Purchase Orders impact Quantity in Stock: 

• When the purchase order status is changed to ‘received shipment’, the quantity in stock will increase.
• When the purchase order is canceled, the quantity in stock will decrease.