I'm confused by the distinction being made between "recipe swapping" and "recipe comprehension/improvement/repairs".
Why are some recipe questions allowed but not others? Aren't they all about cooking?
The most successful questions on this site will have exactly one correct answer. Some questions will still work well with 3–5 answers, but in general, the more answers a question receives, the less useful any of those individual answers become. Voting becomes difficult if not impossible and that means the result is no better than simply going to your favourite recipe site or Google. In fact, it's generally worse.
Questions asking for recipes ("recipe swapping") follow the template of:
Obviously, these questions relate to cooking. However, the nature of these questions makes them actively harmful to a Q&A site:
So instead of trying to explain why every individual recipe request was closed, we have simply designated the entire category as off-topic. If your question is along the lines of "How do I make [name of dish]?" then expect it to be closed for this reason.
Questions about a specific recipe are different. In these questions, the recipe is background information. If you call "tech support", they will usually ask what you were doing when the problem occurred; similarly, if you want to know why your cake came out gritty and dry, it helps us to know how you made it. That's why we not only allow but encourage questions about:
("Why do I need to pre-bake the crust for my pie?")
("Can I substitute brown sugar for white in this recipe?")
("My sauce came out too spicy. What can I do to cut the heat?")
("How can I make french fries exactly like McDonald's does?")
These are all great questions because they'll have a small number of answers and it's relatively easy for an expert to qualify them as correct or incorrect.
Questions about recreating recipes are the most subjective of the above, but as long as they are reasonably specific as to what the end result should be (details about appearance, colour, texture, sweetness, spiciness, etc.) or refer to a very specific and well-known preparation (every McDonald's restaurant makes their fries the same way), then answers can still be qualified and the question will still do well.
The only "recipe" questions we don't encourage (or allow) are indiscriminate requests for recipes, because they don't have right/wrong/expert answers. And to make it easy for everybody to understand that a recipe-swap question was closed because it was a recipe swap — as opposed to being closed because we didn't like it — we use the off-topic close reason.