The changefreq tag may be used by Google to determine how often your pages are updated and so to come back at a ratio of these intervals.

You don't have to include this tag in your sitemap.

The changefreg tag may also be useful to help stop google crawling pages that you don't want googlebot to index repeatedly. eg large PDF files

The lastmod tag explains to Google when you last updated the page, it may use this in its decision of when or whether to crawl your pages.

You don't have to include this tag in your sitemap.

If you do include this tag in your xml all the dates will be set to the date that you specify above - we cannot determine the creation date of your documents at present.

The priority tag has no relation to the current PR status of a page or site. It is included to help you suggest to Google what are the most important parts of your site are.

You don't have to include this tag in your sitemap.

If you choose to use the PR selection the priority will be set to your current PR value in google.

If you use the PR function your page limit is 200 links, I have had to set this because of the server load involved with retrieving the PR of a page.