When doing keyword completion in insert mode |ins-completion|, and 'ignorecase' is also on, the case of the match is adjusted depending on the typed text. If the typed text contains a lowercase letter where the match has an upper case letter, the completed part is made lowercase. If the typed text has no lowercase letters and the match has a lowercase letter where the typed text has an uppercase letter, and there is a letter before it, the completed part is made uppercase. With 'noinfercase' the match is used as-is.