Will TestFlight for Mac be available for apps sold outside App Store?

As I understand it, when you define a custom build rule with a script, Xcode will pass each matching input file to your script though variables like INPUT_FILE_PATH. So you normally don't need to explicitly list input files. However, there is an "input files" section in the build rule definition. How is that used? I'm hoping that it only used for dependencies and deciding whether a rule needs to be run. For example, suppose my rule is compiling a source file. Normally I just want to re-run the compile if the source file changes, but if the compiler itself changes, I probably also want to recompile. Can I list the compiler as an input file?