Package: debian-policy User-level applications should not use sockets below $HOME by default: the filesystem used for $HOME might not support them, as is the case for NFS or OpenAFS.
They should use XDG_RUNTIME_DIR (if set) or as a fallback a temporary directory below /tmp; any location below $HOME should only be used as a fallback. I'm not sure where in policy such a requirement would belong: we don't seem to have requirements for either socket use nor what user-level applications should be doing so far. See also [1]. Ansgar [1] <http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html>