Basically, you can use the companion app for any channel that you have on your Xfinity cable package. There are dozens of tv apps, so it's best that you just look for apps for the channels you watch/like the most. You log in using your Xfinity username and password, so you cannot get into the apps for channels to which you're not subscribed.
Unfortunately, Xfinity does not support single sign-on, so you have to sign in app by app.