The format they use. Find and view a *.desktop file, for instance.

*.desktop files are at most ini-like, they have a different specification that does not mention "INI" at all. What did formerly_proven refer to when saying "XDG ini"?

"INI" is not a specification. All INI parsers are slightly different, such that people refer to any file with

    [Section]
    Key = Value

Format as "INI" or INI-like, including the xdg format.

Heck. TOML is a superset of it.

A subset, any TOML is a valid INI, but not nearly all INI are valid TOML