Change sensitivityIf you're not sure which setting to choose, try the medium setting - it should work well for most websites. You can always change it later if you need to.
The high sensitivity setting is best used when you want to be alerted to any change at all, no matter how tiny - even the slightest change in a website's underlying HTML code will trigger an alert. You may need this setting for web comic pages.
The medium setting mostly ignores the technical structure of the website and concentrates on whether or not there was any change in the readable text on the website. This is a good general setting for small to medium-sized blogs and author indices. It works well on RSS feeds and other XML data.
The low setting is similar to the medium setting, but it's usually more effective at filtering out all the HTML on sites where the HTML is very complex or doesn't follow the rules. It's also less sensitive and it tries to ignore any small changes that probably aren't important to you.
The medium setting might sometimes give a false positive result - it might sometimes alert you to a change when there was no obvious visible change.
The low setting might sometimes not notice a change on a website if it was a very small alteration.
If you are watching for changes in a non-HTML/XML file, e.g. a PDF or a Word document, then use the low or high settings.
The medium and low settings both try to filter out things like numbers and dates.
For a more technical description of what the settings do, check out the technical info page.