"UNKNOWN" - clouds and weather accessories that don't fit any existing category
This link may help to a certain extent with your cloud identification.
That is a fall streak hole. An aircraft has flown through a layer of Ac that is composed of supercooled droplets, and either by the stirring of the air, or by the temperature of the plane, has caused cloud droplets to freeze.
Once this freezing process is underway, the differenced between saturated vapour pressure over water and over ice cause ice crystals to enlarge, at the same time the water droplets evaporate, a process known as the Bergeron Findeisson process, responsible for most of the rain that falls in the UK.
What can be seen is an area where ice crystals are growing, hense the feathery, cirrussy appearance of the ice crystal cloud, surrounded by clear air, where the super cooled water droplets have evaporated. This process is most efficient at temperatures of around -10C.