When an assembly is deleted all assembly data is deleted including design criteria, ply transitions, global plies etc. Component data such as concepts, dimensions, and materials will remain intact.
How to delete an assembly from the Sizing form
In the "Assembly" group of the Home tab, select Edit | Delete.
How to delete one or many assemblies from the FEM Viewer
Color by assemblies.
In the legend, right-click on the assembly you wish to delete.