Цитирует часть выделенного текста в сообщении. Просто выделить часть текста и нажать «Quote» в форме ответа.

Only selected text is quoted as long as it's quoted in the proper quote box.

Hitting the Quote button will only quote the text you have selected, provided the following is true:
a) The selected text contains only text in the forum post you are attempting to quote.
b) The quoted text does not start or end in a quote from another user.
If either of those conditions is not met, the entire post will be quoted, just as if there were no selection.