Unequal angle (UA) is an L-section with one leg longer than the other. Common Australian sizes are 65x50x5, 75x50x6, 100x75x6, and 125x75x8 mm.
UA shines wherever the depth in one direction matters more than the other. A long-leg-vertical UA used as a ledger or jamb gives more height to mount fixings into without making the wall-side leg overly wide. As a brace member, the longer leg can take the higher bending load.
UA is less common than equal angle in residential gate work but standard in commercial and structural steelwork.