Fitted Mesh garments move along the body, same as rigged mesh items do, the garments fitting is significantly better though. Fitted Mesh is not perfected yet and might still need some slight adjustments on your avatar shape. Depending on the animation overrider you are wearing, the alpha mask might still be required.
Rigged and fitted mesh items can be rezzed and (if modifiable) also stretched by using the inworld building tools but as soon as you wear them, they snap back to the original size and position!!
Also due to the rigging, it does not matter the attachment point – they will always attach in the right position.