When navigating to Setup -> Company -> Enable Features, there will not be showing the “Assembly Items” Checkbox under the “Items & Inventory” Tab.
The assembly Item feature is part of a NetSuite Add On module “Light Manufacturing” (also known as Work orders and Assemblies). NetSuite Add-on Modules are offered in bundles specific to the industry and can be licensed separately at any time during the NetSuite contract.
To check whether this module is enabled on the account or not, navigate to Setup > Company > View Billing Information > Add-On Modules tab.
