What do irregular kanji usage and irregular kanji form mean (in dictionaries)?

Question

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)

Answer 1

From what I can see, jisho.org does not seem to be using the term "irregular" strictly, so there may not be a single meaning for that.

As for "風 meaning cold (disease)" and "小供 meaning child", these are rare, nonstandard and/or historical variants. Even though dictionaries may recognize these, they are considered incorrect when you write something in modern standard Japanese. You almost always need to write 風邪 and 子供, respectively. (But note that even 子供 is sometimes considered politically sensitive/incorrect by some.)

Something like 煙草 is called 熟字訓, which is part of perfectly standard Japanese. 当て字 is another way of forming a word. Both are ways of forming a word, but 熟字訓 means "ignoring the reading but respecting the meaning of kanji", while 当て字 means "ignoring the meaning but respecting the reading of kanji". Some "irregular kanji forms/usages" on jisho.org may also be either 熟字訓 or 当て字, but jisho's idea of "irregular" is probably not directly related to these.

And of course there are 異体字 or 旧字体 forms of kanji, which might be also called "irregular kanji form" by some.