Here's what FBSDID("$FreeBSD$"); means and how it works:
Version Information: Like the $FreeBSD$ tag, FBSDID("$FreeBSD$"); is used to embed version and revision information into the source code files of the FreeBSD kernel.
Automatic Substitution: When the source code file is committed to the FreeBSD version control system (usually using CVS or Subversion), the __FBSDID("$FreeBSD$"); macro is automatically substituted with metadata about the file, such as the file's path, revision number, date, and sometimes the author's username.
File Identification: This macro serves as a way to identify the specific version of the kernel source code file being used. It aids in tracking changes, identifying the file's history, and crediting contributors.