XAUAUD is the price of one troy ounce of gold in Australian dollars, derived from XAUUSD divided by AUDUSD, so it moves on both the gold story and the Australian dollar story.
Gold is defensive and the Australian dollar is pro-cyclical, so in risk-off conditions both legs push the same way and XAUAUD trends further than XAUUSD. The quote leg responds to RBA policy, Australian CPI and employment, iron ore and Chinese growth expectations.
XAUAUD is most active in the Asian session, roughly 23:00 to 07:00 UTC. Its spread is commonly two to four times the XAUUSD spread, which makes it a swing and intraday trend instrument rather than a scalping one.