what are you trying to do exactly? next_hudchannel is not an incredibly usefull function so you might be wanting to do something else.
To answer your question this is what it does, as told by the include.
Code:
/**
* Returns the next valid hudchannel for a user, from 1-4.
*/
native next_hudchannel(player);
Code:
new nexthud = next_hudchannel(player)
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