diff --git a/src/daemon/media-session.d/alsa-monitor.conf b/src/daemon/media-session.d/alsa-monitor.conf
index 764af1bc8..954135f12 100644
--- a/src/daemon/media-session.d/alsa-monitor.conf
+++ b/src/daemon/media-session.d/alsa-monitor.conf
@@ -18,9 +18,17 @@ properties = {
 rules = [
     # An array of matches/actions to evaluate.
     {
-        # Rules for matching a device or node. It is an array of
-        # properties that all need to match the regexp. If any of the
-        # matches work, the actions are executed for the object.
+        # Rules for matching a device or node. Each dictionary in this array
+        # specifies the property to match as key and a string or regex match
+        # as value. A successful match requires all dictionary keys (i.e.
+        # properties) to match.
+        #
+        # Actions are are executed for the object if at least one successful
+        # match exists.
+        #
+        # Regular expressions are prefixed with the ~ (tilde) character,
+        # otherwise a standard string comparison is used.
+        # The special value "null" matches against empty properties.
         matches = [
             {
                 # This matches all cards. These are regular expressions