platformio-core/examples/ide/visualstudio/lib

Documentation: http://docs.platformio.org/en/latest/userguide/cmd_init.html

This directory is intended for the project specific (private) libraries.
PlatformIO will compile them to static libraries and link to executable file.

The source code of each library should be placed in separate directory.
For example, "lib/private_lib/[here are source files]".