There is a lot of contradicting information about publishing for Desktop. Some sources say that from 14.10 onwards Click packages also for Desktop can be submitted to USC. Other sources say that Click packages are only for Touch. And then there are Snappy packages.
There are bugs open about submitting free open source apps to USC being impossible (see e.g. https://bugs.launchpad.net/developer-portal/+bug/1409749). I'm SO confused right now. Is PPA still the only way to publish free open source app for Ubuntu Desktop?