xmysite in your Configuration is the variable that holds the URL to your site. It is a very important variable, as a lot of functions in your site rely on it being set correctly.
The format of the xmysite variable is:
http://www.mywebsiteurl.com/vpaspfolder/
It is important that you use the format above, including the / at the end of the line, so that your customers don't experience any errors when shopping in your site.
xmysite is very powerful and can help you achieve all sorts of advanced functions with your site. Some of these include:
1. If you are using SSL certificates and you want to ensure your customers shop using the secure URL, set xmysite to https://www.mywebsiteurl.com/vpaspfolder/ and your customers will be automatically forwared to the secure URL when shopping.
2. If you are having issues with sessions, xmysite can help solve these. Some webhosts would see the following two URLs as completely different sites:
http://www.mywebsiteurl.com/vpaspfolder/
http://mywebsiteurl.com/vpaspfolder/
Setting xmysite to one of these ensures all of your customers, no matter which URL they use to get to your site, end up shopping at the same, consistent URL, meaning no sessions are lost due to any hard-coded links that refer to the other domain.
3. As with the above item, you may have multiple very different URLs that customers arrive at your site through. For example,
http://www.bobsgreencars.com
http://www.bobsredcards.com
Setting xmysite to one of these ensures all of your customers, no matter which URL they use to get to your site, end up shopping at the same, consistent URL, meaning no sessions are lost due to any hard-coded links that refer to the other domains.