If you implement custom fields in your Inventory Records, like Invoice, Quotes, the values will be deleted during any workflows and you couldn’t interact with this values from Workflow Designer before version 600.0825.
With this version, you could create a file named “InventoryFields.inc.php” directly into the “extends” directory.
This file will not be overwritten during updates of the module.

In this file you could configure the fields you create and want to keep during Workflows.
This file MUST have the following structure.
Because this file will be included during every Workflow execution in the Inventory Modules, you should proceed with caution.
Every Error in this file could make it impossible to execute any workflows.

[prism field=file language=php]

[vc_table]tableCol,This%20is%20the%20name%20of%20the%20column%20in%20the%20vtiger_inventoryproductrel%20table.%20Won’t%20be%20used%20to%20read%20from%20database%20%2C%20but%20will%20be%20a%20good%20and%20unique%20name|Testvalue,The%20label%20of%20this%20field%2C%20which%20will%20be%20shown%20on%20task%20configurations|FieldNameWithoutProductIndex,This%20is%20the%20Key%20of%20the%20value%20in%20the%20ProductsArray.%20You%20use%20this%20value%20in%20the%20%E2%80%9CgetAssociatedProducts%E2%80%9D%20function[/vc_table]

This structure will make it possible to also set this fields if you create an Invoice in an Workflow.