Generally speaking, in order to be diagnosed with a mental disorder, the psychological symptoms that you are suffering from need to fall into the definition of something specific. For example, someone with wild mood swings might be bipolar or schizophrenic, depending on the other symptoms that they display. But when someone has several different symptoms that don’t fall into the pattern of any known disorder, they are harder to diagnose.
The DSM-5 changes that for at least one specific subset of patients in this situation by creating a new disorder: psychological factors affecting other medical conditions.