diff --git a/scripts/packextras.py b/scripts/packextras.py
index 0405d608..a5616ec8 100644
--- a/scripts/packextras.py
+++ b/scripts/packextras.py
@@ -746,14 +746,26 @@ def platform_region_groups(
 
 def extra_region_groups(extra: dict) -> list[str]:
     """The region groups a core extra competes in."""
+    emulator = extra.get("source_emulator", "")
     variant = extra.get("variant_group")
     if variant:
         # A variant group is one slot of one core, whatever system each
         # member is filed under: PicoDrive files its US Mega CD BIOS under
         # sega-segacd and the EU/JP ones under sega-megacd, and find_bios
         # picks one of the twelve. Keyed as emulator mode keys it.
-        return [f"{extra.get('source_emulator', '')}:variant:{variant}"]
-    return _extra_system_ids(extra) or ["_extras"]
+        return [f"{emulator}:variant:{variant}"]
+    explicit = extra.get("source_system")
+    if explicit:
+        return [str(explicit)]
+    systems = [str(s) for s in extra.get("source_systems", []) if s]
+    if len(systems) == 1:
+        return systems
+    # No system names the machine this file is chosen for. Filed under every
+    # system of a 30-system profile, CLK's American MSX ROM made the
+    # Enterprise group drop its German ROM. Alone, it drops nothing and is
+    # never dropped.
+    destination = extra.get("destination", extra.get("name", ""))
+    return [f"{emulator}:file:{destination}"]
 
 def emulator_region_drops(
     selected: list[tuple[str, dict]],
diff --git a/tests/test_region_groups.py b/tests/test_region_groups.py
new file mode 100644
index 00000000..2e6c6f9f
--- /dev/null
+++ b/tests/test_region_groups.py
@@ -0,0 +1,67 @@
+"""A core extra competes for a region only with files of its own machine.
+
+CLK profiles 30 systems and names none per file. Its extras were filed in
+the region group of every one of them: its American MSX ROM made RetroDECK's
+Enterprise group drop the German Enterprise ROM under --region us.
+"""
+
+from __future__ import annotations
+
+import sys
+import unittest
+from pathlib import Path
+
+REPO_ROOT = Path(__file__).resolve().parent.parent
+sys.path.insert(0, str(REPO_ROOT / "scripts"))
+
+from packextras import extra_region_groups  # noqa: E402
+
+
+class TheGroupFollowsWhatTheProfileSays(unittest.TestCase):
+    def test_a_file_system(self):
+        self.assertEqual(
+            extra_region_groups({"source_system": "msx", "source_systems": ["msx", "vic20"]}),
+            ["msx"],
+        )
+
+    def test_a_single_system_profile(self):
+        self.assertEqual(extra_region_groups({"source_systems": ["sega-saturn"]}), ["sega-saturn"])
+
+    def test_a_multi_system_profile_without_a_file_system(self):
+        groups = extra_region_groups({
+            "source_emulator": "clk", "destination": "MSX/msx-american.rom",
+            "source_systems": ["msx", "enterprise-64-128", "vic20"],
+        })
+        self.assertEqual(groups, ["clk:file:MSX/msx-american.rom"])
+
+    def test_a_variant_group_stays_one_slot(self):
+        self.assertEqual(
+            extra_region_groups({"source_emulator": "picodrive", "variant_group": "mcd",
+                                 "source_systems": ["sega-segacd", "sega-mega-cd"]}),
+            ["picodrive:variant:mcd"],
+        )
+
+
+class TheEnterpriseKeepsItsRom(unittest.TestCase):
+    def test_retrodeck_keeps_brd_rom_under_a_region(self):
+        if not (REPO_ROOT / "database.json").exists():
+            self.skipTest("database.json is not built")
+        import region
+        from common import load_database, load_emulator_profiles, load_platform_config
+        from packextras import platform_region_groups
+
+        db = load_database(str(REPO_ROOT / "database.json"))
+        profiles = load_emulator_profiles(str(REPO_ROOT / "emulators"))
+        config = load_platform_config("retrodeck", str(REPO_ROOT / "platforms"))
+        groups, _ = platform_region_groups(
+            config, config["systems"], str(REPO_ROOT / "emulators"), db,
+            config.get("base_destination", ""), profiles,
+        )
+        drops = region.resolve_region_drops(
+            groups, region.build_region_index(profiles), ["north-america"]
+        )
+        self.assertNotIn("bios/brd.rom", drops)
+
+
+if __name__ == "__main__":
+    unittest.main()