How are auto CPAP machines different from semi-auto machines?

Automatic CPAP (Auto-adjusting CPAP, Auto CPAP) machines automatically adjust the pressure to suit our symptoms every 1 minute throughout the night. Semi-auto CPAP machines provide constant pressure throughout the night. But will automatically adjust the pressure up and down every 30 hours (or about 5 days) with the adjusted pressure not exceeding +/- 3 cmH2O