"Build active architecture only" setting for ios xcode projecgt

When building ios static library, "build active architecture only" does not affect simulator build, it only builds i386 architecture without any armv7 or armv7s build. However, for device build, if it is set to NO. Then all available architecture will be built (armv7 and armv7s); if it is set to YES, then only the active architecture will be built. But how it decide what is the current active architecture? actually, xcode decides it based on build destination, that is, the connected device or simulator type selectedfrom the build type dropdown listbox.
If the current device is armv7s, then it will build armv7s output. If no real device is connected, then it will build armv7 output, which will also support older device types.