Our auditor can't open the XSD file in the taxonomy folder, what is it, and should they be able to open it?

An XSD file stores its content as plain text in XML format, which means that the file can be opened and viewed with any text editor such as Notepad or Notepad++.

XSD is short for XML Schema Defition, and it is a specification which formally describes the elements and their relationships in an Extensible Mark-Up Language.

The file is used to define what elements and attributes may appear in an XML document. In this case this is a formal description of the iXBRL elements, and as such, your auditor doesn't really need to open it.