Bittering hops are put into the boil, usually, at the start. During a boil complex things happen between the wort and bittering hops, which contribute to the flavour. The things that constitute the most, are the Alpha acids, the Beta acids, and the oils from within the hops. The insoluble Acids are transformed to something that can be absorbed into the wort, The higher percentage of acid value, the Acid content, the more flavour (Mostly bitterness) is absorbed. This generally takes around 90 minutes of a good rolling boil. Some of the oils are lost. Generally, high Alpha hops contain more bitterness.  Acid Absorption, is not an exact calculation, as everyone's brew, is different, everyone's boil is different, so it's only by doing a test brew, can you find out your hop utilisation calculations, for a particular hop type.