Minimal macOS version of macro package: is it for macro code or the code it generates?

Hi, I wonder what's the exact meaning of minimal macOS version of a macro package? Is is for macro code or the code it generates? I though it was the former, but my experiments show the latter is true.

Background: I'm using Xcode 15.0 on macOS 13.6, which can't be deployment target of Swift 5.9 specific code (but it's OK to build code on it). I find it's impossible to develop macro which generates Swift 5.9 specific code (e.g. init accessor) on it, because:

a) To generate Swift 5.9 specific code, I have to set minial macOS to v14, otherwise there would be compile error.

b) If I set minimal macOS to v14, the compile error is gone. However, there is following warning in macro declaration file, which indicate the macro module wasn't built successfully:

External macro implementation type 'DictionaryStorageMacroMacros.DictionaryStorageMacro' could not be found for macro 'DictionaryStorage()'

I know macro needs to be compiled and executed on host. Also it verifies the generated code is valid code. My intuition is that means it needs to build the code a macro generates rather than run it. If so, why generating Swift 5.9 specific code requires setting the minimal macOS to v14? (Swift 5.9 can build code on macOS 13.6, it's just that it can't run them)

Also I know Observable macro (more accurately, ObservationTrackedMacro) generates init accessor. I wonder why I have no problem using it on my MBP? What's the difference? Thanks.