Semantic Linefeed Filling

The term "semantic linefeeds" or "semantic line breaks" refers to a set of conventions for using insensitive vertical whitespace to structure prose along semantic boundaries.

GNU Emacs provides this functionality through two commands: fill-paragraph-semlf and fill-region-as-paragraph-semlf.

The command fill-paragraph-semlf fills a paragraph of text using semantic linefeeds, whereby a newline is inserted after every sentence.

The command fill-region-as-paragraph-semlf fills a region of text using semantic linefeeds as if the region were a single paragraph. Users can set the variable fill-region-as-paragraph-function to the value fill-region-as-paragraph-semlf to enable functions like M-q (fill-paragraph) and fill-region to fill text using semantic linefeeds.

Semantic linefeeds are usually used with markup languages that are not sensitive to newlines when exported to a different format (e.g. Org, Texinfo, Markdown).

Let's say that we have the following paragraph in an Org document:

Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod
tempor.  Incididunt ut labore et dolore magna aliqua.  Ut enim ad minim
veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea
commodo consequat.

After filling the paragraph using semantic linefeeds, the result is:

Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod
tempor.
Incididunt ut labore et dolore magna aliqua.
Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi
ut aliquip ex ea commodo consequat.

However, when exported, in both cases the result is:

Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod
tempor.  Incididunt ut labore et dolore magna aliqua.  Ut enim ad minim
veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea
commodo consequat.

So, what are the benefits?

One benefit is that semantic linefeeds are "diff friendly". For example,

-Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod
-tempor.  Incididunt ut labore et dolore magna aliqua.  Ut enim ad minim
-veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea
-commodo consequat.
+Lorem ipsum dolor sit amet, XXXXX consectetur adipiscing elit, sed do
+eiusmod tempor.  Incididunt ut labore et dolore magna aliqua.  Ut enim
+ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut
+aliquip ex ea commodo consequat.

Versus,

-Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod
-tempor.
+Lorem ipsum dolor sit amet, XXXXX consectetur adipiscing elit, sed do
+eiusmod tempor.
 Incididunt ut labore et dolore magna aliqua.
 Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi
 ut aliquip ex ea commodo consequat.

Semantic linefeeds make easier to spot that the word XXXXX was added in the first line.

Also, they are convenient during code reviews. Shorter diffs and separating ideas with newlines allow to be more accurate when adding comments.

For more details about semantic linefeeds, see https://sembr.org/ and https://rhodesmill.org/brandon/2012/one-sentence-per-line/.