A diagnosis that satisfies all the requirements for that disorder listed in the Diagnostic and Statistical Manual (psychiatrists bible). I’ve had pdocs that give a diagnosis not going by the DSM, but that is super rare to come by.

For a BPD diagnosis by the DSM you have to have at least 5/9 of the following traits:
-Frantic efforts to avoid real or imagined abandonment. Note: Do not include suicidal or self-mutilating behaviour covered in criterion 5.
-A pattern of unstable and intense interpersonal relationships characterized by alternating between extremes of idealization and devaluation.
-Identity disturbance: markedly and persistently unstable self-image or sense of self.
-Impulsivity in at least 2 areas that are potentially self-damaging (e.g., spending, sex, substance abuse, reckless driving, binge eating). Note: Do not include suicidal or self-mutilating behaviour covered in criterion 5.
-Recurrent suicidal behaviour, gestures or threats, or self-mutilating behaviour.
-Affective instability due to a marked reactivity of mood (e.g., intense episodic dysphoria, irritability or anxiety usually lasting a few hours and only rarely more than a few days).
-Chronic feelings of emptiness.
-Inappropriate, intense anger or difficulty controlling anger (e.g., frequent displays of temper, constant anger, recurrent physical fights).
-Transient, stress-related paranoid ideation or severe dissociative symptoms.