I have just noticed this in the BBC delivery spec when using UHD footage:
For images acquired at 50 frames a second vision mixer cuts and edits shall occur so that the start of the first frame of the progressive video pair is aligned to the start of the first (upper) field of an interlace video signal as defined by SMPTE ST2051.
I presume this is to cope with converting to interlaced HD. Does this mean at 50 FPS we should only edit on every other frame? ( a bit like the field sequence on quadraplex VTRS for those of you old enough to remember!)