diff --git a/Help/variable/PROJECT-NAME_IS_TOP_LEVEL.rst b/Help/variable/PROJECT-NAME_IS_TOP_LEVEL.rst
index 953e9783fd5bfd5de83a986a444922847e1d33cd..6718ecfd95a2c5a3c72c02ffb6d5d5071ba08958 100644
--- a/Help/variable/PROJECT-NAME_IS_TOP_LEVEL.rst
+++ b/Help/variable/PROJECT-NAME_IS_TOP_LEVEL.rst
@@ -9,3 +9,13 @@ level ``CMakeLists.txt`` file.
 To obtain the value from the most recent call to :command:`project` in
 the current directory scope or above, see the
 :variable:`PROJECT_IS_TOP_LEVEL` variable.
+
+The variable value will be true in:
+
+* the top-level directory of the project
+* the top-level directory of an external project added by :module:`ExternalProject`
+
+The variable value will be false in:
+
+* a directory added by :command:`add_subdirectory`
+* a directory added by :module:`FetchContent`
diff --git a/Help/variable/PROJECT_IS_TOP_LEVEL.rst b/Help/variable/PROJECT_IS_TOP_LEVEL.rst
index e5eb6c1231ec09b8cdd51702d6635f3aadb654e6..ad61fecc0599ceb1a78ca6309a174086d1d04fcb 100644
--- a/Help/variable/PROJECT_IS_TOP_LEVEL.rst
+++ b/Help/variable/PROJECT_IS_TOP_LEVEL.rst
@@ -19,3 +19,13 @@ options:
   if(PROJECT_IS_TOP_LEVEL)
     include(CTest)
   endif()
+
+The variable value will be true in:
+
+* the top-level directory of the project
+* the top-level directory of an external project added by :module:`ExternalProject`
+
+The variable value will be false in:
+
+* a directory added by :command:`add_subdirectory`
+* a directory added by :module:`FetchContent`