It is a rather simple concept that I have seen in almost all league tournaments.
This is most strongly supported by the recent qualifiers where people would often take the adc and support top. Some games even had solo adc.
almost all champs can do multiple combinations of opponent/ally match ups.
So please, if you call something like "adc" that in no way says you are going to the bottom lane. just means the way you are playing that champ.
If you are wondering where this came from, it came from champions that can fill multiple rolls.
ex: Ezreal, Twisted Fate and Sion: these champs can change roll to ap/ad. It is usually not desired but it happens, hence why these types of calls where introduced.
I just say "not support" at the beginning, because support bores the living hell out of me these days.
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