A package is a combination  of several sevices. There are required  services (which must be used), optional required services (where the user chooses between several services, e.g. several flights) and optional services (which can be chosen). Moreover you determine the order of the services. A service sold in a package can have a price different from the one it has when sold as a separate service. You can also fix a total price for the package only without taking into account the price of the individual services. You define whether a service can be sold in a package only or also as a separate service. In an allocation matrix you can predefine packages in such a way that the user does not have to choose any more between various seperate services such as accomodation types, classes and such like. This is of advantage if you  want to sell packages in the internet.