The Quantity in Stock of a product gets updated in accordance with the status of the invoice or purchase order created for the product. This is done via the workflow UpdateInventoryProducts On Every Save that is set up and active by default in Vtiger.
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.
To learn more about Workflows in Vtiger CRM, click here.
Here are the scenarios under which Invoices impact Quantity in Stock:
Here are the scenarios under which Purchase Orders impact Quantity in Stock: