From a BQL Statement

In BQL expressions, you have to refer to the custom field by its BQL name in the extension class.

For example, after you have added the UsrSearchKeywords field to the InventoryItemExtension class, which is a DAC extension for the InventoryItem class, the field is accessible in BQL, as shown in the code below.

//The usrLocalTaxCategoryID custom field is defined
//in the InventoryItemExtension DAC extension for the InventoryItem class
PXSelect<InventoryItemExtension, 
    Where<InventoryItemExtension.usrLocalTaxCategoryID, IsNotNull>>