Sqlite source code reading. What the ids like F17025, U17032 are used for?

In SQLite source code comments I see identificators like F17025, U17032, etc.

sqlite3.c:

** The sqlite开发者_运维知识库3_mutex_enter() and sqlite3_mutex_try() routines attempt
** to enter a mutex. {F17024} If another thread is already within the mutex,
** sqlite3_mutex_enter() will block and sqlite3_mutex_try() will return
** SQLITE_BUSY. {F17025}  The sqlite3_mutex_try() interface returns SQLITE_OK
** upon successful entry.  {F17026} Mutexes created using
** SQLITE_MUTEX_RECURSIVE can be entered multiple times by the same thread.
** {F17027} In such cases the,

What they are used for? Probably there are some related docs?

I think that these are references to sections of some internal document (think Functional Requirements).