This means the "application class loader" in systemDictionary.hpp:

  static OopHandle _java_system_loader;

This means the "boot class loader" in arguments.hpp

  static void set_sysclasspath(const char *value, bool has_jimage) {
    // During start up, set by os::set_boot_path()
    assert(get_sysclasspath() == NULL, "System boot class path previously set");
    _system_boot_class_path->set_value(value);
    _has_jimage = has_jimage;
  }

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The former is tied to the Java API java.lang.ClassLoader::getSystemClassLoader() so we should leave it as is. The latter should be renamed as the "boot" loader to avoid confusion.