co(1) maintains a list of keywords of the RCS database which are expanded when the working file is checked out. The keyword $Id$ in a document will expand to a string which contains the file name, revision number, the date checked out, the author, the revision status, and the locker, if any. Including the keyword $Log$ will expand to the document's revision history log.
These and other keywords may be used as search criteria of the RCS archive. See the ident(1) man page for further details.