The pose should be natural and beliebable so it looks good.
This may require the photographer to be able to take charge , move the subjects to the proper location and then to set them up in the style the photographer is trying to achieve.
The pose can be kept loose and more natural so it looks like there wasn’t any influence or interference from the photographer. This is a more photojournalistic approach to portraiture. The photographer must be able to see light and use it so it becomes effective in creating this type of portrait.
The other method of posing is more strict in a sense that certain body parts are placed and moved so the pose adds to the composition of the overal photograph.
A certain placement of the arms and hands may create lines that lead the viewer to the center of interest. A tilt of the head and shift of body weight can also enhance the portrait.
A person can be made to look more powerful, passive, outgoing, friendly, etc. by posing them in such a way. This type of posing along with the ability to communicate with the person being photographed is very difficult and may take years to master. Some of the great portrait photographers throughout the world have used this ability in creating stunning portraits that have made then legends.
See Kelly show and explain the importance of posing correctly
See How Yusuf Karsh Posed His Famous Portraits