XAU is the ISO code for one troy ounce of gold, so any currency placed after it creates a gold quote: XAUUSD is dollars per ounce, XAUGBP pounds per ounce, XAUJPY yen per ounce.
Only the first leg is gold. XAUUSD is the tightest-spread benchmark, XAUAUD adds Australian dollar and China risk and is most active in the Asian session, XAUEUR adds ECB policy, XAUGBP adds the Bank of England, and XAUJPY adds Bank of Japan policy, carry unwinds and intervention risk.
Holding two XAU crosses is one gold position at double size, not diversification. Gold can also be traded as COMEX futures (GC 100 ounces, MGC 10 ounces), gold ETFs, or owned as physical bullion.