An mParticle kit is a small library that is specific to the output for your connection. You add a reference to this kit in the mParticle platform SDK that you add to the project for your client app.
Because mParticle supports many output connections, you want to add just the kit for the outputs you use. Kits aren’t needed for server-side forwarding, as the data goes directly from a client app to the mParticle servers before being forwarded to an output.