C++14
16.2 Source file inclusion
A preprocessing directive of the form
#include < h-char-sequence> new-line
searches a sequence of implementation-defined places for a header identified uniquely by the specified sequence between the < and > delimiters, and causes the replacement of that directive by the entire contents of the header. How the places are specified or the header identified is implementation-defined.

A preprocessing directive of the form
#include " q-char-sequence" new-line
causes the replacement of that directive by the entire contents of the source file identified by the specified sequence between the " delimiters. The named source file is searched for in an implementation-defined manner.

If this search is not supported, or if the search fails, the directive is reprocessed as if it read
#include < h-char-sequence> new-line
with the identical contained sequence (including > characters, if any) from the original directive.

https://learn.microsoft.com/en-us/cpp/preprocessor/hash-include-directive-c-cpp?view=msvc-170