I know what an irregular verb is, it is just a verb with irregular conjugation, and they exist in many languages.
I think I know what and irregular kanji reading is: it should be an unconventional reading for the represent character (like 煙草 たばこ).
But what does irregular kanji form mean? What does irregular kanji usage mean? What's the difference between usage and form in this context?
What I'm talking about is, does irregular in this case mean that I should not use the form/using the form is wrong, or that it is used, but I should know it's somewhat incorrect or unconventional? (If it is the latter, what's the boundary between unconventional and "incorrect"?)
For context, on jisho.org, I find 風 (meaning cold) under the first category, and 小供 (こども) under the second one.
For closing I'll just pose on last question: where is 当て字 placed between all of these categories?
(Please correct me on the first two statements if they're wrong, as I am not 100% confident)