Appx
if identityName is specified in your config and differs from your app name, appx id will be changed to identify name.
Linux
Now category is required, because your app can appear under "Lost and found" in the start menu (e.g., in KDE Plasma). If category not specified, mapped mac category will be used or Utility. If now you will get warning about it, but mac category is specified, please, file issue to correct mappings (e.g. file issue "foo it is bar category on Linux, please add such mapping").
macOS for us is a primary OS, and Linux is secondary, so, we don't want to require linux category. Instead, magically compute linux category from mac.