BT.1886 is the"new standard" for SDR. sRGB/REC809 is what came before, and is better suited to news, cartoons, sports etc. BT.1886 is what movies and cinematic TV shows are graded to, so it is best to use it for movies and TV shows. The two were made available to give options.
Since the International Telecommunication Union (ITU) adopted BT.1886 as the recommended gamma function for flat panel displays used in HDTV studio production in 2011, some confusion has arisen over the usefulness of the function, as well as its application in the field.
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Paul