Plugs Introduction
The Plug tool is used to quickly add prepared details to a model.
Selection
In object mode, select a single plug handle, not a plug mesh, and the target model you want to plug into.
If a plug is brought into the scene from a plug library, the handle will be selected automatically.
Surface snapping and alignment will be set up as well.
This way you can use the translate tool g and with it active, hold down CTRL to snap the plug to a surface.
All that is left to do is shift select the target object, followed by running the plug tool.
plug handle and target sphere, which is active