The global auxiliary (VCCAUX) supply rail primarily provides power to the interconnect logic of the various blocks inside the device. In the I/O banks, VCCAUX is also used to power input buffer circuits for some of the I/O standards. These include some of the single-ended I/O standards at or below 1.8V, and also some of the 2.5V standards (HR I/O banks only). Additionally, the VCCAUX rail provides power to the differential input buffer circuits used for most of the differential and VREF I/O standards.
The power supply requirements, including power-on and power-off sequencing, are described in the UltraScale device data sheets [Ref 2].