FreeSurferDev is one of the InternalFreeSurferDistributions. It receives constant updates, which means it always has the newest features, as well as the newest bugs. It is at /usr/local/freesurfer/dev and is used by sourcing the /usr/local/freesurfer/nmr-dev-env file. We recommend that you do not use this distribution unless you have a good reason for doing so.
source /usr/local/freesurfer/nmr-dev-env
See also FreeSurferStable