Figures and toys can have many types of articulation. Two of these types of articulation are ball joints. These can be double ball jointed or single. If what you want is the meat range of movement possible, the best for you would be double ball joint articulation. These types of joints allow rotation and an extraordinary range of movement. These are also really aesthetically pleasing.
As you can see, the mobility on the elbow is almost like the mobility real elbows have, because of this, really varied poses can be made. Although this type of joint are really good, they get loose with time.
The single ball joint doesn’t allow a lot of movement. It can rotate, but it still isn’t very useful. IF what you want is as much range of movement of the figure as possible, single ball joints are not for you. These types of joints are most useful for toys, since they don’t really need a lot of mobility. These joints are also really easy to break.
As you can see Double ball joint articulation have far better articulation than single ball joints.
So if you want to give a gift to a nephew, son or small sibling, the best for you would be to get a single ball jointed figure and if you want a figure for animation or decoration that is able to strike some great poses, you should get a double jointed figure.