• Insert an object to your scene and remember its name. For example you can create a sphere and scale it by a factor of 0.1. If the basic shapes like Cube, UVspehre or Cylinder are too simple you can edit there meshes to form a more sophisticated shape.
• Select your master object in 3D view or outliner view.
• Go to the graph view and press SPACE , select "Sink" and than "Placement" to insert a placement node.

Every placement node is linked to exactly one master object. You can switch this link to an other object. Placement can be used for duplicating lamps or meta objects too. Feel free to experiment.

The data input connection is interpreted as a mask channel. This is a difference to mesh nodes where data inputs channels represents the height at this location. The mask channel is applied after calculating the objects location. You can use this mask to avoid that objects are distributed all over the mesh. For example you made a low polygon mesh representing a tree. The trees master object can be distributed without overlapping using Poisson disk sampling. There is a lake in your terrain and you need to avoid trees standing in that lake. A simple solution is to use the input channel originally created for the mesh as mask channel to filter the objects locations.