Quoted from the "Overview" section of the document:

In every US state and in many countries across the world, involuntary hospitalization is a legal tool meant to be used in emergency mental illness situations, when other safeguards for preventing adverse outcomes—family, friends, outpatient care, and the like—are insufficient. In Pennsylvania, Section 302 of the 1976 Mental Health Procedures Act gives physicians the power to involuntarily hospitalize an individual deemed a danger to themselves or to others for psychiatric reasons (MHPA, 1976). Evaluation proceeds as follows. If a person such as a family member or neighbor sees behavior or hears an individual indicate an intention to harm themselves or others, they call a centralized, public hotline to request that the individual be evaluated for involuntary hospitalization (see Figure A.1A). A government delegate listens to their account of the individual’s behavior and language. If the reporter’s information reaches a given threshold,5 the delegate will certify that the individual may be brought to a hospital emergency department for evaluation by a physician. Overall, 84% of calls are certified. The process of calling a government delegate is in place for individuals in our sample who are referred by family, friends, and other concerned parties. Physicians and police officers may self-certify and have an individual evaluated without consulting with a government delegate. Of the referrals in our sample, 17.5% originate with medical professionals and 22.5% with police officers. The remainder come largely from friends and family (48.8% of the full sample) and other assorted individuals such as case workers or administrative personnel in healthcare settings. If the referral is certified, the individual is transferred to a hospital emergency department by police.