From: Mint on
I have a question about this flag.

Does this mean that old applications can access files only if they are
case sensitive?

I.E. They could open file.txt, but not File.txt

FILE_FLAG_POSIX_SEMANTICS
Indicates that the file is to be accessed according to POSIX rules.
This includes allowing multiple files with names, differing only in
case, for file systems that support such naming. Use care when using
this option because files created with this flag may not be accessible
by applications written for MS-DOS, Windows, or Windows NT.