The DSM-5 grouping for the four "dramatic, emotional, or erratic" personality disorders: Narcissistic, Borderline, Histrionic, and Antisocial. The disorders this site reflects on are all in this cluster.
When a clinician says "there's a Cluster B presentation here," they mean the relationship pattern looks like one or more of NPD, BPD, HPD, or ASPD — usually with overlap between them rather than a clean single label.