extjs 4.1 When do I use a proxy on a store and when on a model?

There are two ways to use proxies in extjs 4.1 (server proxies in my case), one can tie it to a model or tie it on a store.

I am a bit confused about the usage: What is the difference between tieing a proxy to a store, and tieing it to a model?

What situations would one use one way or the other?