The diagonal cloud is actually an example of two of the new cloud classifications. In the upper right corner, it is Cirrus homogenitus, the new classification for a man-made cloud (contrail from an airplane.) In the lower left, the contrail has changed into one of the old classifications, Cirrus fibratus. Because it "mutated" from a man-made cloud , it is now classified as Cirrus fibratus homomutatus.

You can find all twelve of the new classifications on the World Meteorological Organization website at: https://www.wmo.int/wmocloudatlas/ICA-New-classifications.html