To answer your actual question, it does both. It emulates both windows and linux (although linux implementation was done by a contributor and it's probably not as evolved as the windows part). It also does so on every platform, so you can emulate windows on your android/ios phone, even on the web. It cross compiles to pretty much every platform.
It supports various emulation backends, e.g. Unicorn (which uses QEMU under the hood), but also Hyper-V on windows. That's where the sandboxing part comes in to play: As Hyper-V is pretty fast, the emulator starts turning into a sandbox.
Maybe some day I can add KVM support so you can run sandboxed Windows apps on Linux, but I haven't had the time yet. So at the moment, only the slow emulation backends work on Linux.