RSS subscribe/unsubscribe action, query times, and content

[webseo]

Before working on an RSS feed in PHP, I just had a few questions that aren't covered in the basic online guides[*]When you first subscribe to a feed, is there anything telling the script that this is the first sync? IE a tag, a url parameter, or something that conveys to the system that you just subscribed.[*]Likewise, when you unsubscribe to a feed is there anything telling the script that you just unsubscribed?[*]How often is a feed usually queried for changes? Every 5 minutes? 10 minutes? 30?[*]How do you send all of the content? I'm pretty sure its not just one giant file containing everything since the feed was started, but (at least in Google Reader) you see the last 10 or so entries, not everything.Thanks