In this case, I’m speaking about a filesystem that RTEMS presents to the user. The root file system is typically an instance of In-Memory File System (IMFS) when present and other filesystems can be mounted into it if available.
The boot media from which RTEMS is loaded may not even been a filesystem in the traditional sense as it could be a raw offset and size loaded from NOR or NAND (or even from JTAG).