XSL Link option allows to link an XSLT stylesheet uploaded to the site for processing and transforming List View data to which the web part is connetcted.
The input xml passed to XSLT is described here https://msdn.microsoft.com/en-us/library/office/ff602042(v=office.14).aspx
There are two main ways to work:
- Just transform input data and render it as wanted
- Include the default templates and override those as required
When overriding tempaltes:
- To override the template for the headers, use header mode.
- To override the template for the body, use body mode.
- Consider having a look to _layouts (sharepoint server hive folder) main.xsl and internal.xsl to check the templates to override.