diff --git a/pySim-shell.py b/pySim-shell.py
index 871c45e..5aa311c 100755
--- a/pySim-shell.py
+++ b/pySim-shell.py
@@ -48,6 +48,9 @@
 from pySim.ts_31_102 import CardApplicationUSIM
 from pySim.ts_31_103 import CardApplicationISIM
 
+# we need to import this module so that the SysmocomSJA2 sub-class of
+# CardModel is created, which will add the ATR-based matching and
+# calling of SysmocomSJA2.add_files.  See  CardModel.apply_matching_models
 import pySim.sysmocom_sja2
 
 from pySim.card_key_provider import CardKeyProviderCsv, card_key_provider_register, card_key_provider_get_field
diff --git a/pySim/filesystem.py b/pySim/filesystem.py
index 8edac41..fe781de 100644
--- a/pySim/filesystem.py
+++ b/pySim/filesystem.py
@@ -1435,7 +1435,9 @@
 
 
 class CardModel(abc.ABC):
-    """A specific card model, typically having some additional vendor-specific files"""
+    """A specific card model, typically having some additional vendor-specific files. All
+    you need to do is to define a sub-class with a list of ATRs or an overridden match
+    method."""
     _atrs = []
 
     @classmethod
@@ -1456,6 +1458,9 @@
 
     @staticmethod
     def apply_matching_models(scc:SimCardCommands, rs:RuntimeState):
+        """Check if any of the CardModel sub-classes 'match' the currently inserted card
+        (by ATR or overriding the 'match' method). If so, call their 'add_files'
+        method."""
         for m in CardModel.__subclasses__():
             if m.match(scc):
                 m.add_files(rs)