diff --git a/mercurial/revsetlang.py b/mercurial/revsetlang.py
--- a/mercurial/revsetlang.py
+++ b/mercurial/revsetlang.py
@@ -260,9 +260,10 @@
# Constants for ordering requirement, used in getset():
#
-# If 'define', any nested functions and operations can change the ordering of
-# the entries in the set. If 'follow', any nested functions and operations
-# should take the ordering specified by the first operand to the '&' operator.
+# If 'define', any nested functions and operations MAY change the ordering of
+# the entries in the set (but if changes the ordering, it MUST ALWAYS change
+# it). If 'follow', any nested functions and operations MUST take the ordering
+# specified by the first operand to the '&' operator.
#
# For instance,
#
@@ -276,15 +277,28 @@
#
# 'any' means the order doesn't matter. For instance,
#
-# X & !Y
-# ^
-# any
+# (X & Y) | ancestors(Z)
+# ^ ^
+# any any
+#
+# For 'X & Y', 'X' decides order so the order of 'Y' does not matter. For
+# 'ancesotrs(Z)', Z's order does not matter since 'ancesotrs' does not care
+# about the order of its argument.
#
-# 'y()' can either enforce its ordering requirement or take the ordering
-# specified by 'x()' because 'not()' doesn't care the order.
-anyorder = 'any' # don't care the order
-defineorder = 'define' # should define the order
-followorder = 'follow' # must follow the current order
+# Currently, most revsets do not care about the order, so 'define' is
+# equivalent to 'follow' for them, and the resulting order is based on the
+# 'subset' parameter passed down to them:
+#
+# m = revset.match(..., order=defineorder)
+# m(repo, subset)
+# ^^^^^^
+# For most revsets, 'define' means using the order this subset provides
+#
+# There are a few revsets that always redefine the order if 'define' is
+# specified: 'sort(X)', 'reverse(X)', 'x:y'.
+anyorder = 'any' # don't care the order, could be even random-shuffled
+defineorder = 'define' # ALWAYS redefine, or ALWAYS follow the current order
+followorder = 'follow' # MUST follow the current order
def _matchonly(revs, bases):
"""