clusters are just things that share the same problems. example in mental disorders sometimes some mental disorders share the same problems symptoms.

personality disorders have symptoms / problems in common so closely together that they are grouped together. which is why its called a "cluster"

Cluster B (the grouping you are asking about) personality disorders all share the symptoms of being very drama filled / dramatic/ and unpredictable.... example instead of just saying something like I lost my job today, guess I need to hit the classified section of the newspaper, a person with the cluster B disorders would exaggerate and get very dramatic, emotional and suicidal. ..I lost the best job of my life because of some..... who did this that and the other thing what a piece of ... I might as well be .... I will never be able to find another job ..... my life is over.... on and on and on over exaggerating and being extremely dramatic/ emotional.

my point all cluster B disorders share these same symptoms/ problems so they are grouped together.

cluster A is the personality disorders that all share the problem of being paranoid, and cluster C is the personality disorders that all share the problem of fears/ phobias, strange behaviors and avoiding things.