XML is a generic term for various formats used to upload appraisals through a portal. There are different XML formats created by different products.
Appraise-It will support Fannie Mae's new MISMO format when it is required. The MISMO format requirements are available on Fannie Mae's website.
If your client requires XML submissions before MISMO is required by Fannie Mae, you would need to use a 3rd-party format that your client supports, such as AIReady or Lighthouse.