There are two regulators present on the MAX32600, a 1.8V LDO and a 3.3V LDO. The 1.8V regulator derives it's power from the USB rail, if present, or from the main VDD supply and outputs power to the V_REG18 rail to power the digital core. The 1.8V regulator is also known as the retention regulator when the device is in standby mode.
The 3.3V LDO derives it's power from the USB input when present and supplies the VDDB rail. Many different sub-rails derive their power from the VDDB rail when it is present such as the VDDA3, VDDIO, and VDD_REG18 rails.