What does X denotes in ASPX, DOCX, XLSX, PPTX etc?

For the last three, when Microsoft switched to XML-based file formats in Office 2007, they added the 'x' to the file extensions to differentiate from the previous, incompatible formats.

ASPX was added with the introduction of ASP.NET to differentiate from the Classic ASP convention of .asp.

The first is "extended". The others are "XML".

back in the year 2000 asp.net was branded as ASPX end then everything became .NET even BIZTALK server was BIZTALK.NET

The X in Office files denotes that it is now a different storage format. take an Excel file XLSX, rename it to .ZIP or .RAR and then you will see a folder with a bunch of XML files