I think we all know that it is common for D&D and D&D-adjacent rulebooks to be incomplete, in the sense that there are things that have to be done to play the game - such as the GM coming up with heuristic to determine whether or not the outcome of a declared action is uncertain - which the rulebooks don't tell the game players how to do.
What counts as incompleteness can be context-sensitive. Backgammon rulebooks don't normally explain how to roll dice, as it is assumed players are familiar with this - in practice I think many people learn how to roll dice as young children playing snakes-and-ladders and the like. The original D&D rulebooks assumed some familiarity with some elements of play, because they were written for people already familiar with wargaming.
With modern D&D books, though, the incompleteness seems to be clearly for a different reason: the publisher wants to sell the books to as many people as possible, who are playing a variety of different games using a variety of different processes and heuristics, and so refrains from stating any of those in the rulebooks as the way to play the game. It's like a more elaborate version of decks of cards, which can be bought without including any rules on how to actually play a game using them - the manufacturer of the cards want to sell to bridge players and rummy players and poker players.
But even if the rulebooks don't set out a process of play, the actual play of the game requires that some process or processes be adopted. Eg in 5e D&D, the GM must actually decide, in some fashion or other, whether or not the outcome of a player's action declared for their PC is certain or uncertain. For reasons that escape me, though, many posters seem not to want to talk about the actual processes that they or others use.