diff --git a/SKILL.md b/SKILL.md
index 372974d..b2e3912 100644
--- a/SKILL.md
+++ b/SKILL.md
@@ -256,7 +256,7 @@ changes what "migrated" means, so surface it when
 first diff; the test-inclusive configure for [Diff tests](#diff-tests) is
 recorded apart, as `test_configure_command`.
 
-### Create initial BUILD.bazel files
+### Create initial BUILD.bazel files {#create-build-files}
 
 Read `model.<frontend>.json`. For each production target emit a `cc_library` /
 `cc_binary` with `srcs`, `hdrs`, `copts`, `defines`, `includes`, `deps`. Library
@@ -270,6 +270,9 @@ rather than recalled). Put `common --check_direct_dependencies=error` in the
 generated `.bazelrc` so a declared version that MVS overrides fails the build
 instead of being a warning nobody reads.
 
+The reference guide covers what is specific to translating its build system
+(for CMake: the build type).
+
 ### Main migration loop {#main-migration-loop}
 
 #### Extract the Bazel action graph {#extract-bazel-graph}
diff --git a/docs/REFERENCE-cmake.md b/docs/REFERENCE-cmake.md
index ce6227a..db36825 100644
--- a/docs/REFERENCE-cmake.md
+++ b/docs/REFERENCE-cmake.md
@@ -46,6 +46,23 @@ python3 "$SKILL_DIR/scripts/extract_cmake.py" <build> <repo_root> model.cmake.js
 The project flags passed to `cmake` define the platform/options the Bazel
 aquery must mirror.
 
+## Writing the BUILD files
+
+What is specific to CMake when
+[creating the BUILD files](../SKILL.md#create-build-files).
+
+### Build type
+
+`--compilation_mode` is not a translation of `CMAKE_BUILD_TYPE`: Release is
+`-O3 -DNDEBUG` where Bazel's `opt` is `-O2 -DNDEBUG`, and Debug is `-g` where
+Bazel's default `fastbuild` has neither. A project may set its own default
+build type in `CMakeLists.txt` even when none is passed. Read the reference's
+level (`levels` in `diff.json`, or `CMAKE_<LANG>_FLAGS_<CONFIG>` in the cache)
+and pin it in `.bazelrc`, e.g. `build -c opt` plus `build --copt=-O3`. `opt`
+also turns assertions off where the reference keeps them (`ndebug` in
+`diff.json`); `build --copt=-UNDEBUG` undoes the toolchain's `-DNDEBUG`, not
+one in a rule's `copts`.
+
 ## CMake-specific comparison notes
 
 - **Target names** in `target_map` and `exclude_targets` keys are CMake target