— User Option: adaptive-fill-mode

Adaptive Fill mode is enabled when this variable is non-nil. It is t by default.

— Function: fill-context-prefix from to

This function implements the heart of Adaptive Fill mode; it chooses a fill prefix based on the text between from and to. It does this by looking at the first two lines of the paragraph, based on the variables described below.

— User Option: adaptive-fill-regexp

This variable holds a regular expression to control Adaptive Fill mode. Adaptive Fill mode matches this regular expression against the text starting after the left margin whitespace (if any) on a line; the characters it matches are that line's candidate for the fill prefix.

— User Option: adaptive-fill-first-line-regexp

In a one-line paragraph, if the candidate fill prefix matches this regular expression, or if it matches comment-start-skip, then it is used—otherwise, spaces amounting to the same width are used instead.

However, the fill prefix is never taken from a one-line paragraph if it would act as a paragraph starter on subsequent lines.

— User Option: adaptive-fill-function

You can specify more complex ways of choosing a fill prefix automatically by setting this variable to a function. The function is called when adaptive-fill-regexp does not match, with point after the left margin of a line, and it should return the appropriate fill prefix based on that line. If it returns nil, that means it sees no fill prefix in that line.