To Set an Assembly as a Phantom Assembly (AutoCAD Mechanical Toolset)

Click Structure tabTools panelStructure Catalog. Find
Click the Current Drawing tab.
In the Definitions panel in the Mechanical Structure section, right-click the assembly you want to convert.
To convert the assembly to a Phantom Assembly for this drawing only, select Phantom in Current Drawing.
To convert the assembly to a Phantom Assembly in the current drawing as well as in other drawings that xref this assembly, select Phantom.

The assembly is now a phantom assembly and is indicated by the icon in the mechanical browser.