PSoC Mixed-Signal Array operates on a single power supply between 3.0 to 5.25 volts. Analog signals in most systems are typically of both positive and negative polarity around some reference or ground. The PSoC only handles signals of positive polarity with respect to VSS. So, to bring the bipolar input signals within the measurement range of PSoC, an Analog Ground which is above VSS is created and the input signals are referenced to this analog ground.
PSoC has a very flexible Reference selector where different values can be selected for AGND. For example, when the RefMux is set to Vdd/2 + Bandgap, AGND is at Vdd/2, for a Reference selection of Bandgap + Bandgap, AGND is 1.3V etc.