PlatformIO Core is a heart of whole PlatformIO ecosystem and consists of
Multi-platform Build System
Development platform and package managers
Integration components (Cloud & Standalone IDE and Continuous Integration).
PlatformIO Core is written in Python and works on Windows, macOS, Linux, FreeBSD and ARM-based credit-card sized computers (Raspberry Pi, BeagleBone, CubieBoard).
PlatformIO Core provides rich and documented Command Line Interface (CLI). The other PlatformIO-based software and IDEs are based on PlatformIO Core CLI, such as PlatformIO IDE for Atom. In other words, they wrap PlatformIO Core with own GUI.
Note
Please note that you do not need to install PlatformIO Core if you are going to use PlatformIO IDE for Atom. PlatformIO Core is built into PlatformIO IDE and you will be able to use it within PlatformIO IDE Terminal.
Also, PlatformIO IDE allows to install PlatformIO Core Shell Commands
(pio, platformio) globally to your system via
Menu: PlatformIO > Install Shell Commands.