i think youre confused about what an allocating mechanism is. The mechanism itself would be something that helps deal with the fact that there are scarce resources and the only a few people will be able to receive the good. Allocating mechanisms happen naturally in a market through equilibrium prices or must be determined by the firm or government or whoever is in charge of allocating the good.
An example of an allocation mechanism could be the lists people have to be on to receive organ transplants. in this case the allocating mechanism would be the list. Other examples could be new technology that people wait in lines to buy.