#include question - not reading file

[Tranzmetro]

I have a newbie question. I have a file with code in it called test.txt. If I put this in the src directory and put an include statement in the .cpp file, it is compiled. If I put it in the include directory it compiles. If I put it in an external directory and put a search line in the c_cpp_properties.json file using the settings interface, the .cpp finds the file, i.e. the squiggles disappear and I can right click on it from in the src .cpp file and read it, but it isn't found during compile time. Can someone please point me to the documentation about how this works and why it won't compile yet I can display it during .cpp file editing. Thanks for your help.

[Tranzmetro]

I neglected to add that I'm using PlatformIO on Microsoft Visual Studio Code.