Criteria for initiating long-term oxygen therapy (LTOT) in stable COPD patients

What are the current GOLD guidelines criteria for prescribing LTOT in stable COPD? Specifically, what PaO2 and SpO2 thresholds trigger initiation, how many hours/day should it be used, and does it improve survival in all COPD patients or only specific subgroups (e.g., those with resting hypoxemia vs exercise-induced desaturation)?