The implicit directional marks are non-printing characters used in the computerized typesetting of bi-directional text containing mixed left-to-right scripts (such as Latin and Cyrillic) and right-to-left scripts (such as Persian, Arabic, Syriac and Hebrew).
[1] Suppose the writer wishes to use some English text (a left-to-right script) into a paragraph written in Arabic or Hebrew (a right-to-left script) with non-alphabetic characters to the right of the English text.
Without an LRM control character, the result looks like this: With an LRM entered in the HTML after the ++, it looks like this, as the writer intends: In the first example, without an LRM control character, a web browser will render the ++ on the left of the "C" because the browser recognizes that the paragraph is in a right-to-left text (Arabic) and applies punctuation, which is neutral as to its direction, according to the direction of the adjacent text.
in Hebrew, the sentence renders as follows: (Note that in a computer's memory, the order of the Hebrew characters is ב,א,מ,ת.)
This happens because the browser recognizes that the paragraph is in a LTR script (Latin), and applies punctuation, which is neutral as to its direction, in coordination with the surrounding (left-to-right) text.