HCPCS Level II code H2018 denotes psychosocial rehabilitation services billed on a per diem basis, covering structured interventions that support daily functioning, community integration, and recovery for people with serious mental illness. This code matters nationally as payers and behavioral health programs use it to reimburse day-long or full-day rehabilitation programming that is essential for maintaining community tenure and preventing higher-cost inpatient care. Key payers addressed in this analysis include Aetna, Blue Cross Blue Shield, Cigna Health, UnitedHealthcare, and Medicare. Readers will find a concise overview of how H2018 is defined clinically and operationally, what typical sites of service are, and which payers commonly cover per diem psychosocial rehabilitation. The publication also summarizes benchmarks commonly reported for per diem behavioral health services, notes relevant policy considerations affecting coverage and billing, and provides clinical context to clarify when a per diem rehabilitative model is typically used. Data not available in the input will be noted where applicable.