diff --git a/course/models.py b/course/models.py
index 0becff550..575f1590c 100644
--- a/course/models.py
+++ b/course/models.py
@@ -2,7 +2,7 @@
 import json
 import logging
 import string
-from typing import Any, Dict, List, Tuple
+from typing import Any, Dict, List, Optional, Tuple, TYPE_CHECKING
 import urllib.request
 import urllib.parse
 from random import choice
@@ -44,6 +44,9 @@
 from userprofile.models import User, UserProfile, GraderUser
 from .sis import get_sis_configuration, StudentInfoSystem
 
+if TYPE_CHECKING:
+    from edit_course.operations.configure import ConfigParts
+
 
 logger = logging.getLogger('aplus.course')
 
@@ -1113,6 +1116,9 @@ def get_url_kwargs(self):
         # python dicts in single line: http://stackoverflow.com/a/26853961
         return dict(instance_slug=self.url, **self.course.get_url_kwargs()) # pylint: disable=use-dict-literal
 
+    # A ConfigPart object is cached for each instance during an update. Cache is cleared
+    # when the instance is deleted. The edit_course_operations.configure logic is responsible
+    # for setting the cache on update.
     @property
     def config_cache_key(self) -> str:
         return f"instance.config.{self.id}"
@@ -1120,10 +1126,10 @@ def config_cache_key(self) -> str:
     def delete_cached_config(self) -> None:
         cache.delete(self.config_cache_key)
 
-    def set_cached_config(self, obj: Any) -> None:
+    def set_cached_config(self, obj: "ConfigParts") -> None:
         cache.set(self.config_cache_key, obj)
 
-    def get_cached_config(self, default=None) -> Any:
+    def get_cached_config(self, default=None) -> Optional["ConfigParts"]:
         return cache.get(self.config_cache_key, default)
 
 
diff --git a/edit_course/operations/configure.py b/edit_course/operations/configure.py
index 4b9a01507..0bf6f8c30 100644
--- a/edit_course/operations/configure.py
+++ b/edit_course/operations/configure.py
@@ -408,7 +408,10 @@ class ConfigParts:
     the lobject class type.
     - *_keys fields contain whatever the <new> config contained. I.e. they
     contain the set of all such keys. E.g. module_keys contains the keys of
-    all modules in the <new> config.
+    all modules in the <new> config. They can be used to determined whether
+    a module, exercise, etc. still exists in the course config even if nothing
+    has changed (e.g. module config disappears from the modules field in the
+    .diff() result if it had no changes).
     """
     config: Dict[Any, Any]
 
@@ -490,7 +493,8 @@ def get_children(config, n = 0):
         )
 
     @staticmethod
-    def diff(old: Optional["ConfigParts"], new: "ConfigParts"):
+    def diff(old: Optional["ConfigParts"], new: "ConfigParts") -> "ConfigParts":
+        """Return a new ConfigParts object containing the differences between the old and new configs"""
         if old is None:
             return new
 
@@ -502,6 +506,7 @@ def diff(old: Optional["ConfigParts"], new: "ConfigParts"):
             keep_unchanged=True
         )
 
+        # Get the changes between the configs
         categories_config = get_config_changes(old.categories, new.categories)
         if categories_config is None:
             categories_config = {}
@@ -550,8 +555,11 @@ def configure(instance: CourseInstance, new_config: dict) -> Tuple[bool, List[st
         errors.append(_('COURSE_CONFIG_ERROR_MODULES_REQUIRED_ARRAY'))
         return False, errors
 
+    # Get the previous config (the one used in the last successful update)
     old_cparts = instance.get_cached_config()
     new_cparts = ConfigParts.from_config(new_config, errors)
+    # Get the changes between the new config and the previous config with some auxiliary information.
+    # See ConfigParts and ConfigParts.diff for more info
     cparts = ConfigParts.diff(old_cparts, new_cparts)
 
     config = cparts.config
@@ -761,6 +769,7 @@ def configure(instance: CourseInstance, new_config: dict) -> Tuple[bool, List[st
                         for child_key in lobject_config["children"]
                     )
 
+            # Delete or hide learning objects that are not included in the module anymore
             for lobject in outdated_lobjects:
                 if (
                     not isinstance(lobject, BaseExercise)