ACL's main bearings are made of a softer metal so that you don't wear out your crank or engine block. Additionally, a thin layer of oil lubricates the connection between the bearings and the crank allowing the crank to "float" and reduce wear. Proper support via main bearings is crucial to prevent oscillations and wear that can damage the crankshaft and other components as the motor moves further out of balance.
Subaru main bearings are specific to either a #5 or #3 journal bearing position. Journal bearing position is not VIN or year specific, it must be known prior to purchase.