diff --git a/FAQ.md b/FAQ.md
index d9cf6c6..d3e2d25 100644
--- a/FAQ.md
+++ b/FAQ.md
@@ -60,13 +60,13 @@ In Emacs, it is:
 
 ### Project root detection
 
-[emacs-ccls](https://github.com/MaskRay/emacs-cquery/) locates the project root with `ccls-project-root-matchers`:
+[emacs-ccls](https://github.com/MaskRay/emacs-cquery/) locates the project root in the following order:
 
 * `.ccls-root`. If this file exists in any parent directory, that directory is treated as the project root.
-* `(projectile-project-root)`. Then this function is called. You likely don't want `/usr/include/c++/8/algorithm` to be treated as in the project `/usr/include/c++/8/`, `(setq projectile-require-project-root t)` inhibits the behavior.
+* `(lsp--suggest-project-root)` which in turn calls `(projectile-project-root)`. Usually you don't want `/usr/include/c++/8/algorithm` to be treated as in the project `/usr/include/c++/8/`, `(setq projectile-require-project-root t)` inhibits the behavior.
 
 The root directory is sent to ccls (the language server) through the `rootUri` field in the `initialize` request.
-ccls finds `.ccls` or `compile_commands.json` in the directory.
+ccls reads `.ccls` or `compile_commands.json` in the directory. If neither exists, ccls fallbacks to `%clang $path`.
 
 ```
 proj
@@ -80,7 +80,7 @@ proj
 
 ### Maximum number of file descriptors
 
-When indexing ccls itself, some files require more than 1000 file descriptors. Remember to increase `RLIMIT_NOFILE`.
+Some projects may require more than 1000 file descriptors. Remember to increase `RLIMIT_NOFILE`.
 
 ```zsh
 ulimit -n 32768
@@ -143,6 +143,37 @@ ccls will save a file in `cacheDirectory`:
   ...
 ```
 
+### Diagnostics in headers
+
+For efficiency, headers are compiled on their own (as if `clang [options] a.h`) to utilize Clang's preamble optimization.
+
+For headers that are not self-contained (e.g. some `/usr/include/bits/*.h` which require a predefined macro) ccls emits diagnostics that you don't see when building the project.
+
+```c
+// a.c
+#define M
+#include "a.h"
+
+// a.h
+#ifndef M
+#error M not defined
+#endif
+```
+
+One approach ccls explored before was to compile its import file `a.c`, but it caused severe performance issue for some projects.
+For smaller files this does not matter and checking for a header guard may help the situation.
+
+### Multi-version headers
+
+A header can be included in different translation units with different compiler flags. It may behave differently via `#ifdef`.
+Ideally there should be some mechanism to switch among different views (compiler flags) when you are editing the file.
+
+For efficiency, a header is only indexed with one set of compiler flags, so either the `#if` block or `#else` is indexed, the other is left blank.
+
+For macros, there is code to track different usage, thus you may find them able to jump to multiple targets.
+`index.multiVersion` is an experimental option to index every version to certify that the author is aware of this issue :)
+It may not be very useful though. It is of type `number` but not `boolean` as the author is also aware of other dimensions of the multi-version problem.
+
 ### Definitions
 
 `textDocument/definition` can be used in many places. Some are current implementation details and may subject to change.