Abstract: Instead of an application specifying that it uses an entire API, the application specifies the subset(s) of the API that it uses. Specific hosts can choose when to implement a subset of the API set without having to support other subsets of the API. When the host implements a subset of API set that was not previously supported, an application that specified the use of the newly supported subset begins to work on the hosts automatically. An application may specify subsets having different versions. For example, the versions of different subsets that are specified may be different. When the host supports the subsets used by the application, the application is activated (i.e. “run”). When the host does not support one or more of the subsets used by the application, the application is not activated.