diff --git a/mercurial/exchangev2.py b/mercurial/exchangev2.py
--- a/mercurial/exchangev2.py
+++ b/mercurial/exchangev2.py
@@ -147,6 +147,9 @@
             for mark in cset.get(b'bookmarks', []):
                 remotebookmarks[mark] = node
 
+            # TODO add mechanism for extensions to examine records so they
+            # can siphon off custom data fields.
+
             # Some entries might only be metadata only updates.
             if b'revisionsize' not in cset:
                 continue
diff --git a/mercurial/wireprotov2server.py b/mercurial/wireprotov2server.py
--- a/mercurial/wireprotov2server.py
+++ b/mercurial/wireprotov2server.py
@@ -397,6 +397,8 @@
         'framingmediatypes': [FRAMINGTYPE],
     }
 
+    # TODO expose available changesetdata fields.
+
     for command, entry in COMMANDS.items():
         caps['commands'][command] = {
             'args': entry.args,
@@ -474,6 +476,8 @@
 def changesetdata(repo, proto, noderange=None, nodes=None, fields=None):
     fields = fields or set()
 
+    # TODO look for unknown fields and abort when they can't be serviced.
+
     if noderange is None and nodes is None:
         raise error.WireprotoCommandError(
             'noderange or nodes must be defined')
@@ -563,6 +567,9 @@
             revisiondata = cl.revision(node, raw=True)
             d[b'revisionsize'] = len(revisiondata)
 
+        # TODO make it possible for extensions to wrap a function or register
+        # a handler to service custom fields.
+
         yield d
 
         if revisiondata is not None: