The different types of paths in Windows are local file system (LFS), such as
C:\File.ext, uniform naming convention (UNC), such as
\\Server\Volume\File.ext, and Long UNC or UNCW, such as
One of the differences between LFS and UNC versus Long UNC is that LFS and UNC are limited, depending on the Windows version and/or individual API, to either 260 bytes in ACP format or 260 codepoints in UTF-16 format (or 260 characters in UCS-2 format). Long UNC is limited to 30,000+ codepoints in UTF-16 format (or 30,000+ characters in UCS-2 format) and must be in Unicode.
ACP stands for "ANSI Code Page", which is somewhat of a misnomer since none of the Windows code pages are actually ANSI compliant. The Windows operating system specifies a particular code page as the ACP, which then is used as the default / fallback code page for the 'A' suffixed path APIs. The ACP routines have the shortcoming in that they cannot represent all file paths that are representable in the operating system; they can only represent the character subset of Unicode UCS-2 paths which have characters in the particular code page's character set.