Empty Marker is not a constraint, it's a helper component to help developers to select GameObjects which're hard to visualize and select by usual means;
(If you're familiar with the other 3D-softwares such as Blender/Maya/etc, you should know about such helper objects already)
See the preview image above, usually we cannot select the "spine" or "neck" transform by clicking the model in the scene, it will cycle between the root GameObject & the one with SkinnedMeshRenderer(SMR). An alternative is to select the transforms in the hierarchy window or load the selection with Ctrl+Alt+number.
EmptyMarker would be very effective and easy to use in such cases. Want to select the spine transform? just click the RectFrame mesh in the above image.
EmptyMarker is a great partner for constraints and IK system. Check the image below (click to open gif).
These markers are automatically disabled & hidden when in game-mode & runtime, so you don't need to worry about they disrupt the scene setting or increase the performance cost.
* How to add the EmptyMarker?
1. Navigate to the target GameObject, add the "EmptyMarker" MonoBehaviour. e.g.: Hips;
2. Click the "Presets", select one from the popup list, recommend the "RectFrame";
(You could use custom meshes too, just put the mesh file under the directory "Assets/Skele/Common/Res/Marker/Models". Then when you press Presets, the new model will be found and shown in the list)
3. When the GameObject with EmptyMarker is selected, the mesh will be tinted yellow. Now click it again to navigate to the marker's mesh GameObject (which is named as "mesh").
4. When you select the mesh object, you could move/rotate/scale it to make it easier to select and handle later.