Mesh Deformer
This deformer (also known as a Cage Deformer in other programs) allows you to create a low-resolution cage around the object to be deformed, which can then be used to freely deform the object. The deformer behaves a little like the FFD deformer, but you can freely define your cage objects.
What can this functionality be used for? Well, for a lot of things..
For example, the deformer can replace a larger joint/weighting assembly if you just want to carry out a few quick deformations. But it can of course also be used in connection with joints when a low-resolution mesh controls a high-resolution mesh (which may be difficult to weight). You then simply apply weights to the low-resolution mesh, which in turn deforms the polygon monster.
Since the cage object is polygonal, any other deformer or expression (such as Pose Morph) can also act on it.
The Mesh Deformer can also work in addition to the joint deformation by additionally deforming the high-resolution mesh (controlled by the joints) using the low-resolution cage (which follows the mesh movement). Deformations can then be added at any desired position. This allows very complex compression and expansion effects to be realized, for example.
The Mesh Deformer can also be used as a modeling tool, for example to quickly (and non-destructively) change some basic character proportions. This is very practical if you do not yet know exactly in which direction your character should develop or if a fickle customer is not exactly a paragon of decisiveness.
There are many possible applications, experiment!

Example workflow for using a Mesh Deformer:
- Create a Mesh Deformer and assign it to the object to be deformed in the Object Manager.
- Create a second object (the cage) whose points define the deformation.
- Drag the cage object into the Cages list of the Mesh Deformer. Click on the Initialize button.
- Select the cage in the Object Manager and switch to Points mode. If you now edit these points, the original object will be deformed.