To Add Selected Geometry to a Custom Content Library (AutoCAD Mechanical Toolset)

In the drawing area, select the geometry to add to the content library.
Right-click and select Save as New Content.
Note: The Save as New Content menu option is not available if the selection set contains geometry inserted from a content library.
In the Content panel of the Save Content As dialog box, select the folder you want to add the item to.
In the Save as section, specify a name for the new item.
In the View section, from the View list, select the name of the orthogonal view you are adding.
If the geometry represents is a sectional view, from the Type, list select the type of sectional view.
If you are not creating a sectional view, select None.
Click Save Content.