cyrus-cyr_synclog(8) on Linux
The text 0f9f21b38eb8, as Debian 10 (Buster) shipped it.
Cyrus IMAP documentation is used to append a value to the log file. You can either specify the log type and value, or an entire log line.
Flags it lists
-
-C -
-M -
-U -
-a -
-b -
-c -
-m -
-n -
-q -
-s -
-u -
-v