A very powerful feature of Emulab is its ability to load custom OSes (disk images) to its nodes. In order to provide control over the testbed nodes, Emulab software uses network-booted, memory-filesystem-based FreeBSD systems (hereafter known as MFSes). Each node upon powering on will boot one of these images over PXE/tftp and check in with the Emulab control system. At this point the node may either remain in the MFS, boot an existing partition on its disk, or receive a new disk image from boss via the control network.
If you are in the middle of installing a new Emulab, the installation process has already downloaded and localized the MFSes for you, but if you need to update later (say, if your hardware does not boot and you need a new kernel from Utah), then you will want to look at the instructions on the MFS Import page.