Target shapes, also known as blend shape targets or just targets, are the individual "poses" that you create in order to deform the base object. The offsets of vertices from the base object in its original state are saved in each target shape as delta information.
You can add target shapes to a blend shape deformer in different ways.
To add a target shape, do any of the following:
An empty target shape is added to the selected blend shape deformer. The target type is chosen automatically as appropriate to the selected blend shape deformer - see Add Blend Shape Target Options.
The Edit button for the new target shape is automatically selected (turns red) so that you can immediately modify the base object's geometry into a new shape for this target shape - see Edit target shapes.
If you add a new target shape while in Edit mode for another target shape, the editing switches to the new target shape.
Once you have created target shapes, you can edit them to make each one unique - see Edit target shapes.
Adding target shapes using this method does not create a separate (target) object for each target shape you create. However, if you want to have actual geometry for a target shape, you can do that as described in Create a new object from the target shape.
If you're using multiple target objects to deform the base object, a target shape is automatically created for each target object you picked when you created the blend shape deformer for the base object - see Create blend shapes using multiple target objects. You can also add more target shapes as described below.
To add empty target shapes
An empty target shape is added to the existing target shapes for the blend shape deformer.
If you want to have actual geometry for an empty target shape, you can do that as described in Create a new object from the target shape.
To add a target shape based on a target object
You can then edit each of the target shapes, which lets you modify the geometry of the corresponding target object - see Edit target shapes.