É um derivado do Utopic retomado em agosto do ano passado. O seguinte email detalha isto. link
"We will then have an "ubuntu-phone" (name TBD) distribution in
Launchpad with a series named something like "rtm-14.09", and will
land changes for RTM there, allowing other parts of Ubuntu to move on
without risk of breaking phone images. "
Assim, o arquivo usado pelo telefone é um derivado do arquivo usado por volta do desktop Ubuntu 14.10.
Mais detalhes no link
"The point of this approach is to make sure that we're completely
confident in the infrastructure and have no unforeseen glitches, without
imposing an undue load on engineering teams while lots of people are
still busy with significant feature development."