From your own data 19% of patients need hospital care. Hospital care is usually required when the patient develops significant shortness of breath. Shortness of breath implies hypoxemia. Hypoxaemia is caused by a disturbance of gas exchange in the lungs. All of these patients have pneumonia with the early CT changes of upper peripheral parts of the lungs showing ground glass opacities which then spread to show more extensive ground glass changes with haloing and areas of consolidation which merge into a whiteout of the lungs.
The Chinese CDC report has a slightly different definition
The authors of the Chinese CDC report divided the clinical manifestations of the disease by there (sic) severity:
Mild disease: non-pneumonia and mild pneumonia; this occurred in 81% of cases.
Severe disease: dyspnea, respiratory frequency ≥ 30/min, blood oxygen saturation (SpO2) ≤ 93%, PaO2/FiO2 ratio or P/F [the ratio between the blood pressure of the oxygen (partial pressure of oxygen, PaO2) and the percentage of oxygen supplied (fraction of inspired oxygen, FiO2)] < 300, and/or lung infiltrates > 50% within 24 to 48 hours; this occurred in 14% of cases.
Critical disease: respiratory failure, septic shock, and/or multiple organ dysfunction (MOD) or failure (MOF); this occurred in 5% of cases.
The decision to move from high flow oxygen (nasal cannula) to masks and then ventilation (proning) is based on standard protocols for viral pneumonia. ECMO is used when even ventilation is unable to maintain satisfactory oxygen levels.
Covid-19 has an odd characteristic of patients with mild hypoxemia suddenly compensating and needing to be rapidly intubated. As a side note, intubating these patients is one of the most risky of procedures for medical personnel as it places them into very close proximity to a very sick patient who is expelling lots of virus.