1. A system comprising: one or more processors; one or more computer-readable media; computer-readable instructions on the one or more computer-readable media which, when executed by said one or more processors, cause said one or more processors to implement a method comprising: describing one or more software extensions using descriptions, the extensions being configured for incorporation in a software platform executing on a client; and delivering the descriptions of the one or more extensions to the client via a network, the descriptions containing code for implementing, without downloading the extensions, at least part of a functionality of the extensions and being configured for use in downloading the software extensions via the network; said acts of describing and delivering being configured to enable software to be delivered over the network, wherein delivery of the software is performed by delivering software files in accordance with a file download priority determined by scenario runs, wherein file usage statistics collected from the scenario runs and one or more priority levels associated with the scenario runs define a download order that reflects files that will likely be used by a user first.