By standard, when we click on ‘create workorder’, then corresponding item is displayed in the workorder.
This is a standard field ‘assemblyitem’ and it’s field help is mentioned below
Select the Assembly you need to build.
After an item is selected, the assembly components are displayed on the Items subtab.
This field cannot be changed after an assembly build is associated with this work order.
The corresponding item’s component is described below.
This component is displayed in the Workorder item tab.