Level of Personality Functioning (LPF) represents the entry criterion (Criterion A) of the Alternative Model for Personality Disorders (AMPD) in the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5). It is defined as a dimensional general severity criterion common to all personality disorders and conceptually independent of personality types or traits, and it represents maladaptive self (identity and self-direction) and interpersonal (empathy and intimacy) functioning. We review the history, measurement, and significance of LPF. We show that the inclusion of LPF in the AMPD is well justified if it is defined as a general adaptive failure of a subjective intrapsychic system needed to fulfill adult life tasks. If so defined, LPF distinguishes itself from maladaptive traits (Criterion B of the AMPD) and captures the contribution humans make as agentic authors to the interpretation and management of the self. While Criterion B maladaptive traits provide important descriptive nuance to manifestations of personality pathology, maladaptive LPF is conditional to the diagnosis of personality disorder.